If they were no beeps at all when you turned it on, it means that it failed to POST (Power-On Self Test).
This usually means that something big is going on, such as a faulty PSU, that the RAM or CPU aren't properly placed, things like that. Is there any chance that something might have happened while you were away? (ie. a power surge, someone accidentally hitting the case, etc).
If the answer is no, open it up, clean it all with compressed air, and check all connections, and try again. Sometimes what causes this is not the PSU itself, but a faulty connection with the motherboard.
If all else fails, try to borrow a different PSU and try.