QUAD CORE INTEL XEON MAC PRO RED DIAGNOSTIC LIGHT STAYS ON CONTINUALLY NO PICTURE

Hi

I have a QUAD CORE INTEL XEON 8 GIGS RAM running 10.6.8 only because I like it and it's stable for the music production I do . But my problem is yesterday my computer shut down by itself and rebooted never did this before . I then went to use the computer and it was like it went into sleep mode and that is when I saw the RED light glowing from inside and it sounded like the optical drive was continually reading a dvd I had in the tray . I tried to get it out of sleep mode but nothing worked so I held the power button in and the unit powered down but the RED light on the mother board right behind the power button stayed on even after I shut down the computer it stayed glowing RED . I then pulled the side door and pulled the optical drive tray and put the hardware test disc in . then powered up the mac pro and I noticed a RED light come on for a little while on the processor tray . when I started up the mac pro I held down the "D" button and the screen flashed from black to grey a few times and then a hazy grey / black screen for a few seconds then a screen with a small computer type icon with a measurement bar and then it went to the Hardware test window where it was checking the computers hardware the bar got to about half way after being on for about 10-15 minutes and then dropped out into a sleep type mode just like yesterday when I first tried to use the computer after it rebooted itself . the Red diagnostic light still stayed on the whole time and has not gone off until I unplugged the the whole computer . I purchased the computer new in 2009 . can anyone give me some ideas to get a little farther so I can diagnose this problem or tell me what the heck is wrong with my computer . I can fix anything on it I just need to know what is wrong

thanks Andy