Computer Screen Froze, Turned Off and Won't Show Up On Monitor

I found this computer on the side of the road about 6 months ago and it has been running fine until now. There is no model number on the case so I think it might have been a PC someone built. I do know the motherboard model which is the ASUS M3A78 motherboard.
The screen froze and wouldn't respond, so I turned the computer off. When I try to turn it back on all the fans spin and the computer appears to be turning on normally but nothing shows up on the screen. It's not a problem with the monitor so it has to be the computer. I turned it on with the case off and noticed that on the back of the video card, which is the ASUS EAH4870 Radeon HD 4870, there are three lights labelled D1603,D601,D602, that flash three times and turn off. I don't know if this is significant but I'm including it just in case. The power supply is the ANTEC Neo HE550 and it is 550 watts.

Steps I've tried:
Removing the CMOS battery for 5 minutes
Reseating the RAM
Reseating the graphics card
Holding the power button for 30 seconds

Is there anything else I can try to get it to turn on?
Help would be greatly appreciated.

Edited by LightlySalted, 16 February 2017 - 11:48 PM.

Well if you found it on the side of the road I would assume it might have had problems beforehand.

 

Do you hear a post beep (one beep) after it starts? If not then the MB might be on the way out. If you do hear a post beep then I would pull the video card and use the video port on the MB and see if you get your screen back.

If that is the case then use the video on the MB or replace the video card. 

There is no beep code, I've tried another card but still nothing shows up. There is no video output on the MB itself.

Ok, no beep at all would probably mean the MB is shot or it is not getting any power.

But as you said you do see some lights coming on (the video card) I would still be thinking MB.

As a last resort if I had an extra power supply laying around I would try to swap that.