Programs lag or can't run entirely
My programs take an extremely long time to load or crash constantly.
Failing or inadequate memory
Replace the RAM modules or add more sticks of RAM. Newer programs utilize more system memory and since this is an older machine the stock RAM might not be adequate.
Failing or inadequate graphics card
Check the graphics card to see if it's running properly, if it's not replace the graphics card with the same or better model.
Mac Pro won't turn on
I push the power button, but nothing happens
Failing power supply
The power supply might have gone out. Remove it from the computer and investigate for any damage replace if necessary.
Mac Pro won't boot
The Mac Pro turns on, but I don't see the home screen.
Corrupted Boot drive
Not much can be done for a damaged hard drive. If desperate take it to an expert, if not consider replacing the hard drive or getting an SSD which would drastically improve performance.
Turns on but no response on monitor
I'm seeing dots, lines, discoloration, or broken images on the screen. Or nothing at all
Failing Graphics card
The graphics card is what produces the image on the screen. If you can't see anything on the screen the graphics card is likely the cause. Consider an upgrade for better performance in all programs especially games.
Disk stuck in optical drive
I try to open my disk drive, but it won't open.
Remove optical drive
Remove the optical drive and carefully open it, remove the disk, close it, and re-install in the Mac Pro.