monitors and USB stopped working

I just installed a liquid cooler in my PC and everything seems to be running fine but my monitors and my USB ports. The GPU fan is spinning so I know it's getting power as well as all the drives and cooling system. I tried losing up the screws on the cooling system, thinking it was causing a short but that didn't help. I also tried hooking the old CPU fan back up (without putting it on the CPU or disassembling the cooling system ) but that didn't help either. Thoughts?

Did you forget to connect the second power cable? The motherboard has two, a big one and a little one. If you forgot the little one, you'll get basically that exact symptom -- everything spins, it all looks normal, but nothing actually happens. This is because the CPU never actually starts, not having the power it needs from the secondary connector.

I did check that. I wonder if I need a new power supply though. I think I only have a 650w

The next most likely explanation would seem to be some kind of installation error when you remounted the motherboard (I assume you pulled it out.) If that's what you did, pull it out of the case again, and see if you can get it running on the static bag it came in, or on a piece of cardboard. If it works there, you just need to remount it correctly.

If you didn't actually remove the board completely, then look very, very carefully to see if you knocked a screw under the board or somehow shorted something out with the CPU backplate or front mounting. And check that all your RAM is in tight, and that any expansion cards you have are firmly seated.

You might have just randomly had something major die on you right exactly when you did a big project with it, but the chances of that aren't very high. It's probably something you did, and hopefully you can fix it without having to replace anything.

If everything is still dead after all that, then the next thing to do is probably to start swapping pieces into other PCs, if you have any, to try to determine which part or parts is/are failing.

Oh, a bum PS is possible, but 650 watts is usually fine for the great, great majority of PCs. What CPU and video card do you have?

I guess I should have mentioned that I also bought new RAM that I thought was the same as my current. Because once I pulled the old RAM out everything started working fine. I do get a CPU fan error on boot but the fan seems to be running fine. I don't know how to make that stop but I think it's because I don't use same power input as the old fan. Any idea where I can shut that off? I checked the BIOS but didn't see any obvious options for shutting it off.

Normally, that will be in the fan control area in the BIOS, usually a big dedicated screen. There will be some kind of 'minimum alert level' or something worded vaguely like that. It means 'below this many RPM, throw an error'. Since your cooler is now on a different plug, you should be able to set the minimum warning speed for the CPU fan to 0, or 'disable', or 'ignore', or something like that.

A normal air cooler radiates heat from the CPU directly into the air, and the amount of heat it can shed is dependent on surface area and how much air is moving across those surfaces. Most motherboards have the ability to speed the CPU fan up as the radiator gets hotter, so that it can shed more heat and keep the CPU from throttling.

A liquid cooler is almost exactly the same thing. All it does is move the problem away from the CPU, but once the heat gets to the radiator, it's got the exact same problem that an air cooler does, shedding heat into the air, just in a different spot. If you've got one of the normal coolers, their radiative area is maybe smaller than a big air cooler, so it's extra important for the CPU fan to be responsive to increasing temperatures. If you plug into a chassis connector, the system may still be able to speed it up some, but it won't do it until much later than it really should.

Now, if you got something like one of the big Koolance units, those have a huge liquid reservoir with a monstrous radiator and a special temperature sensor, so in that case, you don't need to be connected to the CPU fan. But if it's a little one, like a Corsair H60, or one of the Intel watercoolers, moving that connector is probably a bad idea. You are, most likely, seriously impairing the ability of your cooling system to handle high CPU loads.