Computer turns on but no signal.

Hello everyone. I am having a problem with my parents' pc.

The computer in question is mounting an asrock Alive NF6P and the main problem is that even tho it turns on, it doesn't show much on the screen.
At times (but now it's been a while) I have luck getting something on screen keeping pressed with no power on the power button ( I heard it resets the bios ) . But after that it wouldn't work properly, the keyboard didn't work for example. And whenever I restarted I didn't have signal on the monitor again.

I thought the problem was the PSU (both the old one and new one are 500w) so I changed it with another one which was correctly functioning ( I was using it before I bought a new one) but the problem is still there.
I thought the battery in the mobo had drained off but changing it had no effect.

I even tried putting on an old GPU (but completely functional, I tested it) an ATI 2600 but still no signal.
Last time I got some signal it got stuck in the startup with the following error: "CMOS checksum error".

Now I am completely stuck and can't get any signal anymore.
I am suspecting a mobo failure. Any ideas?
Thanks for the help.