Another computer on this network has the same ip

Check both computers are set to DHCP-assigned, then reboot everything.

You can use the command prompt to type

ipconfig /all

that will show your ip.

This is a common issue if you have manually set your IP addresses. As the above users suggests, resetting everything(computers/routers/ect) will force all systems to request a new IP if they do not have a manual IP setup

To check if you did this, you can
Enter the Control Panel
Select Network and Internet
Select Network and Sharing Center
select Local Area Network
In the Window that opens select Properties
On the next page select Internet Protocol Version 4(TCP/IPv4) and select Properties.
Ensure that Obtain an IP Address automatically is selected. Same goes for DNS
Select Alternative Configuration and make sure it is set to Automatic Private Address
Ok out of all those Windows and see if things are better.

If you are a Visual Learner, View this image

Turn off everything, wait a bit, turn it all back on.

Even have the router and cable or dsl modem off?

Check what IP address each device is on.

You could try setting everything to a manually-assigned address (make sure they are all different but in a valid range).

If you have lets say.

192.168.1.24 with a subnet of 255.255.255.0(common in a home network), you have 192.168.1.2-192.168.1.254 that you can use. chances are 192.168.1.1 is your router and 192.168.1.255(i think, been a while) is your broadcast(so you will not be using those.).

The same place that my image shows allows you to setup your own IP, but they HAVE to be in the same network as your router to allow internet access without any messing around.

But maybe it is a better idea for us to get an idea of exactly what your network setup is.

What modem, router and other hardware is on the network.

My computer often loses connection when it falls asleep and wakes up. Today the entire network was not working. Fixed by removing the batteries from the modem.

Modem? Batteries? Not something I've seen before. Who's modem/what kind?

Your DSL modem. You should have a box. When you unplug it, if it holds some lights on, this means there may be batteries inside. Look for them or "reset" button. In my case "reset" did not help, taking out batteries helped. If you do have batteries, unplugging modem will not clear or reset it. My is cable, and I also disconnected the cable connection.

Nope, never seen a DSL modem with batteries. Sometimes they can hold up for a second or two off the caps in the plugpack, but that's it (and I've got a lot of DSL modems/routers lying around).

I never used DSL, all my modems were cable and they all have batteries. Not your typical AAA or AA, but plastic box that hidden inside.

That may be it - there's no cable where I am. I do have a Motorola Surfboard from my grandparents who do though, so I might have a look in that.

The batteries on many Cable modems are because they also offer an option for phone service. This allows phone functions(corded or cordless with battery backup in the base) in a power out. At least that is the only ones like that I have seen.

One less thing on my UPS

Oh - where I am there's still twisted pair for the phone even if there's cable.

That explains it.

Your twisted pair phone system works in a power out because the provider has backup power on its end Personally I think that idea works great(no need to worry, they tend to have the power to keep things running far longer compared to the modem battery). These VOIP setups the cable companies have are just much cheaper for the amount of features you get.

Actually, I have naked DSL and VoIP. But there's a 100Ah SLA going under my floor in a couple of days, which should last a router and a raspberry pi for a while.

What will be interesting is how they handle the fibre rollout, because they're using the twisted pair as a draw-wire. I believe the current advice is 'get a UPS'.

That is exactly what I did. This way i can shut down in power out without data loss and still use the cordless phone(yeah yeah....blahh blahh cell phone ) if needed.

I guess we should stop the thread jacking. Sorry....

Yeah.

@OP, try manually setting the IP address of the device where you got the error to something like 192.168.1.20 (if your router is on 192.168.1.1, if not just get a reasonably high address in the right subnet).

As said previously the battery is for the phone so it should not affect the router even if it is unplugged.

Just the fact that this helped is suspect.

For the IP address conflict... Even cable modem routers from a company will have an accessible web page. Look on the bottom or back of the modem for an IP address that looks something like this 192.168.0.1.

Type this IP address into your web browser (Internet explorer is probably what you are using). A web page will be displayed this is a local web page from your routers firmware/software.

Find the tab that says networking or LAN or something to this effect. Every router manufacturer has a different layout.
Everytime you connect a device to the router the router assigns it an IP address. Once the device or computer is connected the IP address is saved for that device in the routers memory. Even if the device is disconnected from the network. When you reconnect the device to the network the router recognizes it based on its MAC address.

MAC address is the address that is permanently embedded in your device memory. More accurately it is the number assigned to each network adapter. So if you have a computer with an ethernet plug and a wifi adapter you will have two MAC addresses. One for each.
The MAC address will look something like this - 2E:AD:GF:3E:7B:9B

If in the CMD prompt on your computer you type ipconfig/all you will see a listing of all your network adapters. The MAC address is also called the Physical address.

So back to your problem. On the router web page look for a list of all the assigned IP addresses and see if there are two computers with the same IP you may also find this information in the LOG file of the router.

If there are you have to release them essentially you will be deleting their MAC addresses from the Routers memory. Once this is done to ensure it does not happen again uncheck any box on the web page that says "assign ip automatically" and then it should give you an option to assign them Manually and then begin following the advice above. being sure to stay within the parameters of the router 192.168.0.2 thru 192.168.0.254 or what ever your IP address is.

PS since you can not see your WIFI to connect to it you will have to plug a computer directly to the router via ethernet cable.