Windows not recognizing the 3.5mm audio jacks? (Speaker icon greyed out)

I'm guessing that, when your system was not in use for a while, you had disconnected all the speakers, etc. Then you plugged them all back in when re-connecting everything. This may have caught you in a problem with the way Realtek systems implement a way to re-assign output ports.

First of all, check in Device Manager that you do have the Realtek device divers installed OK. Then in Windows go through Control Panel to Sounds and Audio devices. There you can find a drop-down list to select the Default Sound Playback device (Windows can only use one of these at a time) and ensure it is set to the on-board Realtek system.

Now you'll need to do some work to set up that audio system. Arrange your equipment so you can reach the plugs and jack panel on the back of the computer, and also see your monitor and reach your keyboard and mouse for simple things. I'll describe the sequence I have to use - yours should be similar.
1. Unplug ALL the speakers, headphones, microphone, etc. from the audio jacks. Reboot your machine so it knows that there is nothing plugged into any jack.
2. Now plug in ONE thing - say, the front L/R speaker plug into the light green jack. Watch your screen. A small window should show up saying that it has detected that something was plugged into that particular jack. It will suggest what it thinks that thing might be, and ask you to confirm or change. If you do nothing, this window will disappear after a time, but it will NOT use the default setting it had suggested - it will simply send out NOTHING on that jack! So you MUST answer by clicking on the right thing and confirming, and then the window will go away.
3. Next, plug in one more thing and answer the query to tell it what that was. Each time you do this, the Realtek system learns what output device was plugged in where, and directs the correct signals out that port.
4. Keep on doing this one at a time until you have plugged in all your speakers, headphones, mic, and whatever.
5. Now try using the Realtek configuration tools to set what speaker system you are using, and test the output. It should all work.