Computer won't boot after upgrade

Hello,

I have just installed a new PSU and CPU fan, this involved removing the MOBO to fit the fan mounting. Upon restarting the machine the power light on the MOBO comes on and the fans all start up but after about 5-10 seconds they all stop and restart, once theyve restarted they stay on but no beeps and no signal to my monitor (have tried both monitors).

I have tried anything I could think of, swapping the new parts out for the old, removing/swapping the memory, trying my other grpahics card, still the same problem.

This is my first time installing something more complicated than a HDD, RAM or GFX card so I was extremely careful to follow all instructions and safety guideline when doing this kind of thing.

Any help would be appreciated!

Thanks