I have a custom built tower computer and when i turn it on I can hear the fans working but nothing shows up on the screen. The power button works to turn on the computer but then, even if i hold it, in an attempt to turn it off nothing happens. So I have to turn it off using the main switch in the back. This happened a few days ago and then resolved itself without me doing anything and is now happening again.
The power fan can running. You should check all the cables, HDD cables on the motherboard, especially the Memory, plug out it and re-plug it to another slot. The showing problem may caused by the memory.