So, in a nutshell, after two USB peripheral failures and an attempt at a reboot my computer now does absolutely nothing when I hit the on button. I've tentatively identified the likely culprits as either the PSU or the motherboard.
I had the notion that if it was the motherboard that had died that the PSU fan should still spin up when powered on. If the PSU fan doesn't spin up, then the PSU is the likely culprit.
I swapped the PSU out with a spare I have and plugged the replacement into the mobo with both plugs (20 pin and 12v). At this point when putting a system together I usually flip it on for an early test and get the PSU fan and a mobo beep. Running this test now, however, shows the exact same issue. It does absolutely nothing-no PSU fan, no beep from mobo. The blue "I'm alive" light on the mobo is lit up, however, so I'm not quite sure what's going on.
Welcome to the forums, Newcomer. First, click on the link in my signature. Work through the checklist of common areas to address for "no boot" / "no display" concerns. Note that at the end of the list, there is a link to jsc's breadboarding guide. Should the standard checklist be unhelpful, consider using the breadboarding guide to rule out case shorts.
If after consulting both guides, you still have the problem, post back with any new observations. Good luck.