Computer doesn't start

Hello. I have an issue with my computer. Anytime I attempt to start the computer, it makes a noise as if it's returning from standby/sleep mode and nothing happens. I get a black screen, and it won't go any further. No windows start up screen, none of that. This recently started happening. The first time it happened was when I actually made it go to sleep mode and later tried to get it back but it nothing happened. I have tried resetting and reconnecting everything. Maybe something is not connected properly in the CPU, but I have opened it up and checked. It might be due to the graphics card and that it's not receiving enough power supply? A few days ago a nvidia error came up that said the graphics card was not receiving a proper supply of power or something. I didn't think that was a problem since I read that many people got the error and nothing happened. I will look over my manual and see if I have everything in place.

Here are my basic specs, I will need to look again to see what psu and motherboard I have.

computer: emachines T5234
graphics card: nvidia 9800 gt

Do u have warranty,If u have contact the dealer!