[Troubleshooting] just swapped out computer case, suddenly computer doesn't want to work

Ok, so just today I switched out my computer case from a thermaltake V4 to a beautiful Bitfenix Colossus (the white one with a window) on arrival the case was dented in the top but that wasnt anything a bit of elbow grease couldnt fix. But now that im done building it, I went to turn it on and it starts up just fine, but soon enough, it randomly shuts down. I attempt to turn it on again and it fails to do so. For the last 30 minutes ive bee trying to get the damned thing to turn on but for some reason the only thing that will go on is the HDD activity LED. I suspect that the problem is the PSU, but then again, that same PSU was working in a different case only a few hours before, and the only things I added on to its load are three fans and 2 LEDS, so what could be going wrong?