hi, I built a new PC with a q6600 at stock and an 8800gt, 2gb RAM Asus ds3r mobo etc. Last night it was working perfectly but this morning when i turned it on, my monitor doesn't get a signal, and my mouse, (razer lachesis) lights do not come on, neither does my crappy dell keyboards. I also noticed that the PSU fan is not working. All of my case fans work when I turn the power on and I've tried switching gx cards to no avail. I just really want to know whether a dodgy PSU could cause these problems or whether you think it may be something else. Thanks a lot for your help guys. Ant.  

Did you plug in the 4-pin power connector?
Clear the bios.
Some PSU fans only work when there is heat.
Please list your specs. We can help you better.

my first thought was psu.. what brand is it and what are the specs on it?.... no lights at all  and no fans usually points to the power supply...

We need your rigs specs,? also what kind of PS ?

the brand of the PSu is antec smartpower 450w. it was in my old rig and has been working in my new one for quite some time. when you say Did you plug in the 4pin? DO you mean to the GFX card? If so Yes I did. All my case fans work when i turn on the PC, as does the fan on the gFX card. It's just the fan on the PSU that is not working.  
 
Specs: Antec 450 w psu
Q6600 quad core cpu at 2.4ghz
2gb corsair ram
en8800gt gfx
2 dell hard drives
p35 ds3r mobo

Ya I dont think it has enough power although its a good brand. You say its old so the power has degraded over time.
 
The guy wit hthe 4 pin needs to read the whole post. Disregard that. I'm sure he was talking about the 4pin CPU connector. If you didnt plug it in you wouldnt of played the night before.

I removed the RAM from the motherboard and didn't get any beeps or anything. Is it likely that the PSU could power all my case fans fine but not enough to produce motherboard beeps. I also cannot see any lights on the motherboard.

 
 
 
450W is plenty for that computer.
 
 
I have similar specs and had an issue like that one weekend.  Came home when computer was idle and couldnt get any picture.  I was drunk when I came home and tried a few things and couldnt get it to work.  Swapped out vid card with a known working one, took out memory, nothing.
 
Next morning I took the entire computer apart and put it back together (i didnt remove the cpu/heatsink) and turned it on and problem solved.  Must've been a loose wire or a short somewhere.  I would suggest doing the same since you said it was working.

Usually the +5v rail goes, and causes your symptoms. I see this alot in my shop.

Like roadrunner said, most likely your PSU is chocking for not being able to supply enough power. A 450W PSU is usually the minimum recommended by most manufacturers to use with a 8800GT board, but it's never a good idea to use the minimum.  
 
To test the theory you could try to turn your system on but with only the minimum hardware required for it to power on attached to the PSU. Disconnect all hard drives, optical drives, floppy, case fans (but do not disconnect the CPU fan) and any other peripherals that consume energy. Do not install any extra PCI cards you might have (like a sound card). Only install the video card. If you have more than 1 stick or RAM, only use one.  
 
Try to turn the system on this way. If it comes on and it has video signal most likely it is the PSU not being able to supply enough power when the entire system is assembled.  
 
If it still doesn't turn on properly, you still cannot rule out that the PSU has just gone bad and in this case you might have to get another PSU to test.

If it's a SmartPower, it is a lousy power supply. My first one lasted 3 months. My second lasted 1½ years. If it's a SmartPower2, it's better.

OK, I hope it is the power supply, it does make a rather odd very high pitched whine for a second or 2 when i turn it on now. What I really need to know though is if it is possible for a power supply to be able to power about 5 fans but when all fans are disconnected it still can't power the motherboard. Does the motherboard use a different rail? I don't know very much about PSUs as you can probably tell :-) But is it possible that it cannot supply the voltage for the mobo but has no problems supplying the voltage for the fans? That would tell me that it is the PSU. Thanks a lot guys. Ant.

Try to take one module of RAM out and then start it up, it had the exact same problem with my PC

I've tried taking one module of ram out and still the same, tried taking both out and no motherboard beeps. Does anyone know if a PSU could break in such a way that it can power 5 fans but not a motherboard?

 
Yes, it is possible.  My wife's computer also had a similar problem last week. When we tried to power it on, some fans came on and we could hear a high pitched whine too, but the system would not power on. I replaced the PSU with a spare I had and that fixed the problem.
 
You almost certainly have a defective PSU unit. My advice is to replace it.
 
 

Your power supply is way to small for that setup, you're running 5 fans and a 8800 you need at least 550watts. The 12volt rails are whats important. I'm betting your under 20amps and thats the problem.

OK thanks a lot guys, i'll go and replace my PSU tomorrow and hopefully that'll fix it :-)