The fact that your system operated for a time after the BIOS update indicates that the update was functional. If your PSU was found to definitely be faulty, and replaced, it's probably this event rather than aything to do with the BIOS update that is causing your troubles.
Is the replacement PSU known to be working correctly, all connectors fully seated to correct locations (none overlooked), and have you checked to see that nothing else was dislodged during the PSU fit (eg a memory DIMM moved)?
Try clearing the CMOS memory (with the system unpowered, either by moving a CLR_CMOS or similar jumper on the motherboard, or by removing the backup battery for some time). Sometimes due to a static zap when a system is worked on, the CMOS memory can contain an invalid state and set an unbootable condition. (This is likely to give your symptom, fans operate but nothing else happens.)
There's also a possibility the original faulty PSU may have damaged something, in which case it's a process of elimination to find what. If nothing else corrects the fault condition and the new PSU is proven to operate correctly in another system, the usual approach is to disconnect all peripherals from the motherboard (cards, drives, memory but leave any external beeper/speaker so you will hear any POST beeps) and start the system. You should get insistent beeping due to the absent memory. If not, either the CPU or motherboard is not operating correctly (or both...). If you get beeping, things can be added one by one beginning with memory then the video card etc until you find what stops the system from powering up.
Always remember to ground yourself by touching the metal case whilst working, and have the system fully powered down and turned off at the mains (not just soft-off) when changing any components.
Edited by Platypus, 15 November 2007 - 11:57 PM.