|You'd temporarily disconnect all the drives. All you want to do is get something to show on the screen. The posting screen or system logo comes up before an operating system loads. You don't need to have any drives connected for that or any cards like modem, network or sound. As long as you reconnect everything correctly afterwards you won't do any damage--well, any additional damage given some hardware item is probably bad.|
The voltage selector is usually a small red switch on the back of the power supply near where the power cable is connected. I just mentioned it because I've run across a few where the switch was in the wrong position. If no one has been fiddling with your computer then that's not going to be the problem.
But it's possible the power supply itself is getting weak and it may be the problem.
It could be one of the cards, one of the drives, the cpu, (probably not the ram as you'd get a beeping error then), the power supply, a short in one of the data or power cables or some motherboard failure. Based on its symptoms, there's just no way to know for sure. The best you can do is rule out things by disconnecting them and then seeing if it makes a difference.
Just recently someone brought me a dead PC. By the above process of elimination, I found both its power supply and motherboard had failed. I didn't know what specific part on each was bad but that didn't matter because neither would have been practical to repair. So I replaced both and reinstalled all the other original parts and it worked fine.
If you're uncomfortable fiddling around in the case you might want to find someone more knowledgeable about computers to help. You don't want to end up making things worse. I'm just saying the above procedure is the best non-technical way of getting started.