Just built PC won't start

[Lunix89]

For some reason, despite doing everything correctly, my newly assembled PC won't turn on. So I took everything out of the case and checked the cpu was installed correctly, and fit it all together and wired it up outside the case, and then it DID turn on. It worked. Then I uwired it, put it back in the case, wired it up exactly how I did before and now it won't turn on again. Just because I put it back in the case. I am 100% positive I put the wires in the same exact place as I did when they were outside the case. this doesn't make any sense at all, can someone please help? Thanks.

 

[alexoiu]

Check the standoffs if in the right spots. Does the CPU fan spin when starting the board?

 

[Shadowbi]

For a computer to turn on it only needs power to the (functioning) motherboard. Everything else, if wrong, may cause the computer to not boot, but the lights and fans should at least turn on and, if the beeper is connected, it should beep, possibly communicating an error code. If it really is not turning on, then possible problems are:
1 - The power supply cables are not connected properly, either being in the wrong spots, not connected, or not connected all the way in;
2 - The power button wires are not connected properly to the motherboard;
3 - The cable that connects the PSU to an outlet is not connected properly;
4 - The PSU or motherboard are damaged and not functioning properly;
5 - The cables or connectors are damaged;
6 - There is a short circuit somewhere (good PSUs don't turn on if this is the case);

For the connections, there are 2 from the PSU to a modern ATX motherboard: a big one with 24 pins and another with either 4 or 8 pins (if 8 pins, then it is split into 2 connectors with 4 pins each).
The most probable in your case is that due to the cramped space in the case you couldn't get the connectors all the way in. Failing that, check if there is any metal touching the MB (it should touch only the screw standoffs, which may be metallic with no problem, as long as they are in the right spots, which are only where there are screw holes on the MB), and if the cables from the case (including the power button) are in the right place, since it can be hard to connect them when the MB is inside the case.
If you already did all that, then give more details of what you did, such as what cables you disconnected and connected again when you moved everything outside the case and back again.