This happens off & on: My Gateway 5458E desktop will turn on, but the POWER light doesn't come on (should be blue) & there is no signal to the monitor. The computer powers up, can hear sounds (fan, etc), keyboard lights up, can see light from DVD player, but nothing on monitor & no power light. Have tried a different monitor with same results.

The computer has also turned itself off while working ok a couple of times & wouldn't restart correctly until next day, but most of the time this happens when I try to do a restart or just turn on computer after being turned off.

What could be causing it to start sometimes & not others?

Thanks!A failing Power Supply sounds like the root cause of this issue.

Any local computer store will be able to help you with one. The power supply is one of the most important parts of a computer.
If they OFFER something REALLY cheap, eg $30 it is not very good quality, the higher quality the better.

Assuming you are using the machine as standard, a 350W PSU will cover you, however if you plan on getting a graphics card to play games or video edit ect a 500W minimum should be followed.