You don't mention this, so it is unlikely, but it COULD produce your symptoms. IF you have a floppy drive, and IF it is set as one of your early boot devices, and IF you accidentally reversed the way its data cable plugs into the back, you'd get this result. The boot sequence would go to the floppy drive but get nothing but communication errors, so it would keep trying and lock itself into a loop, without ever finding a place to boot from.
Just as another thing to try, check the jumpers on all your IDE devices. In fact, just in case one of them is making poor contact, remove and re-insert each of them (several times each) one at a time. A clash of device identities on an IDE channel can cause errors that produce a failure to boot.
I also like amnotanoobie's suggestion of dead PSU.
By the way, the former performance that it beeped once as it completed its POST is the normal signal that there were no problems encountered with hardware during the POST. The fact you get no beep at all right now says it has not found a hardware problem during POST so far, but it still cannot complete the boot sequence. In fact, it may not even be finishing the POST process since it fails to give you that "all-OK" single beep.