It's a voltage, temperature or fan alarm.
Possible a lazy fan - or you left a cable interfering with a fan when you were inside the case
Or your power supply is now a touch too small to cope, so you have a lower DC voltage than you should.
You need to do a visual check inside the case, and also (if you can get it to power on) look at the DC volts in the BIOS and see what is out of kilter.
A good check to see if it's a power low function, is to unplug the optical drives and see if it goes away.