Need some Tech support

Turned my pc off normally last night, there were no problems at all. Heard a little "pop" sound some time in the middle of the night and when I went to turn it on this morning, the power button did nothing.
I switched out the power cable and now the box powers on, but there's no output to the monitor. Sounds like it's booting though, but I'm not sure.

This is what I've done so far:
checked everything is clear and undamaged inside the case, it's pretty clean and there's no sign of burnt out parts or anything
tried different power cables
tried different monitor, monitor cables and both VGA slots
tried a much older graphics card (which might not be the best test, since it might be broken)

I am thinking it must be something wrong with either the graphics card or the motherboard now? or something gone on the PSU and it's not supplying enough power maybe.
Kind of out of ideas for what this might be or what I can do to fix this, so if you know anything about hardware I could really use some advice.