get a new CPU fan?
Or, hopefully you just dislodged the fan power wire which plugs into the motherboard. Check it's plugged in. If it's not you should see a socket for it on the MB near the fan's position.
Seems quite likely if you were changing the PSU that something isn't plugged in right.
Other than that I'm pretty clueless abut computers...so...get a new CPU fan?
