Is it possible for you to swap out the PSU or test it?
With this kind of problem I would start there especially as you say everything has been running fine until now.
The other thing to check is that everything is seated properly. Make sure that all the mobo connectors are pressed home properly (it seems stupid but even without moving a PC around they can come loose).
If you have tried this then it becomes a little harder as you will need to try and find out what else is causing the problem. My second port of call would be the graphics card (usually continuous long beeps show if there is a fault here). Take it out and make sure the contacts are clean and look OK then re-seat it.
If the problem persists then do the same with the memory. Take out all the sticks and replace them one at a time (rebooting with each individual stick)
I know it is frustrating but don't try and do all of this at once. Do one thing at a time until you can find the fault, or at least narrow it down.
