Computer only shows blank screen. No BIOS

I have DELL desktop computer that's already modified quite a bit. I've changed the graphics card and the power supply. I usually run Ubuntu on it. There's a Windows partition, but I've never used it.

Everything worked. I never tued it off, except some times to restart it. The new power supply does not fit in the old case, so I have to keep the case open with the power supply on top. I didn't mind but it did get dusty in there.

One day, I got back home and the screen was blank. I restarted the computer. Still blank. I tried vacuuming inside (an old trick) but it was still blank. I tried removing one of the two pairs of RAM bars, still blank. I put the RAM bars back in and removed the other pair. Still blank. I tried a different graphics cable. I tried another screen. I replaced the graphics card with an old one. Still not even a BIOS message.

I really, really, really don't want it to be a motherboard issue. Is there a clear way to diagnose the issue as coming from the motherboard? But at this stage, other than the motherboard, what else could it be?