Well, as I said, it can still be dead. All you can do is try resetting the BIOS, and jump starting it with the power button connection removed from the mobo header, and jumping the two power button pins with a small screwdriver or paper clip.
Reset the BIOS with the AC power cable removed > remove the BIOS battery > press the power button and hold it pressed for a minute > wait 10 minutes before putting the battery and AC cable back on... I wouldn't give you high hopes but those are the last tricks to try before giving up.
As for the PSU you may want to try the PC with another one.. it may also have suffered damage and may not supply enough amps to powerup everything besides the HDDs which are directly connected to the PSU.