Hi Cindy,
Try resetting the SMC.
To reset the SMC on a Mac Pro:
From the Apple menu, choose Shut Down (or if the computer is not responding, hold the power button until it turns off).
Unplug all cables from the computer, including the power cord and any display cables.
Wait at least fifteen seconds.
Plug the power cord back in, making sure the power button is not being pressed at the time. Then reconnect your keyboard and mouse to the computer.
Press the power button to start up your computer.
Another way to reset the SMC on a Mac Pro computer is:
From the Apple menu, choose Shut Down (or if the computer is not responding, hold the power button until it turns off).
Open and remove the metallic outer door.
Press the SMC_RST switch that is located slightly below and to the right of the row of diagnostic LEDs. Use a non-metallic, non-conductive object, such as a wooden pencil. Note: Mac Pro (Early 2008) does not have an SMC_RST switch, refer to the reset procedure steps in the previous section.
Replace the metallic outer door.
Press the power button to start up your computer.
From here:
http://support.apple.com/kb/HT1806
Carolyn 🙂