Compaq DeskPro starts up but won't display on monitor?

Hi,

I've got an older computer here that I'm trying to get back up and running as a project, but I don't know a whole heap about computers so just trying to learn

It starts up as in, the LED starts flashing, it makes beeps and I can hear fans going etc, but it will not display anything on my monitor. I have tried 3 different monitors with 2 different cables each, and just to be sure it's not the monitor, I have plugged those in and turned them on too and they do display, just not for the desktop. Just a note too because I can't remember any computers I had doing this, but it always starts up as soon as I plug it into the wall without me needing to press the button on the front, I'm not sure if that's worth mentioning/part of the problem.

Internally I have had a look going off other troubleshooting websites and it doesn't look like there is any physical damage like leakages or blown capacitors, and it looks pretty clean too for its age.

I have heard that common issues relating to this are videocard errors, RAM issues or a bad CPU, but I wasn't sure which to investigate first or whether there is something else I am better to check/try first.

If anyone knows what I could try I'd be grateful for the advice. If you do need anymore info just let me know and I will update - thank you