PC display gets no signal. What should I try next?

I cleaned some dust from my desktop PC today (I thought I was being super careful, didn't touch or remove parts except side panels), but after plugging everything in, the screen no longer gets signal. It searches fir HDMI and analog signal for a moment, then goes to sleep.

I can hear Windows starting up, so the computer's not completely dead. All fans appear to be spinning; case fans, power source fan, CPU and graphics card fan.

I have tried the following:

1. Unplugged the PC and screen power cables, held PC power button down for 1 minute, plugged back in. Didn't work. (Tried twice with 30 seconds then 1 minute)

2. Plugged the same HDMI cable and screen to a laptop. They work fine.

3. Plugged the screen to the PC using VGA cable, still no signal.

Not sure what to try next. Is there any way I can test whether it is a graphics card, mother board or power issue? Or just something needs resetting? Unfortunately I don't have any other graphics cards or other spare parts at hand to try.

Any help is greatly appreciated.