Monitor displaying teared image with red lines and low resolution (620x480) after 6 months off

Well, hello everyone. I'm pretty inexperienced when it comes to PC maintenance, so I hope you guys can help me.

My PC spent the last 6 months turned off and unplugged. Today, when I tried to turn it back on, the screen was completely teared, with lots of interference, some red lines and low resolution (although It did loaded Windows and I could see my desktop, but it was almost unreadable). This has happened before (I guess 1 year ago), so I removed my GPU, cleaned the dust and used some contact cleaner, same as I done before. But, this time, I didn't worked.

I then plugged my monitor directly on the motherboard's HDMI and it worked perfectly.

So, my question is, is there any way to know it the problem is the GPU or the PCI-E slot on the motherboard? I don't have with me any other GPU that are working.

Thanks in advance to everyone.