Check your Users Guide. The Mac Pro 2010 version has this to say:
If the status light flashes and the computer doesn’t start up
Â If you recently installed memory, a PCI expansion card, or another internal
component, make sure that it is correctly installed and that it is compatible with your
computer. See if removing it allows the computer to start up.
Â If the status light still flashes, determine the number of flashes to identify the problem:
• One short flash followed by a longer off period: No valid memory.
• Three short flashes followed by a longer off period: Failed memory.