You are not providing much information to work with. What model Mac Pro are you using? Determine that by typing its serial number in the search field found on the following page: https://selfsolve.apple.com/agreementWarrantyDynamic.do
To determine its serial number read Mac Pro: How to find the serial number
One obvious cause would be a programmed shutdown time in System Preferences > Energy Saver > Schedule...
If you programmed no scheduled shutdown time, you may be able to determine what is causing your Mac to shut down. Find the Console app - it is in your Utilities folder and looks like this:
You can find it by selecting Utilities from the Finder's Go menu.
If the log list column on the left is not already displayed, show the log list by selecting Show Log List from Console's View menu.
Locate system.log in the list and select it. Type the words previous shutdown in the Filter field at the upper right of the Console window. Is there a number associated with the log entry for Previous Shutdown Cause at the time your Mac Pro unexpectedly shut itself down?
Power or other problems related to unexpected system sleep, shutdown, lights or fans justify an SMC reset. Read all the steps.
Before Resetting the SMC
Try each of the following steps in this order before you reset the SMC. Test the issue after completing each troubleshooting step to determine if the issue still occurs.
- Press Command + Option + Escape to force quit any application that is not responding.
- Put your Mac to sleep by choosing the Apple () menu from the upper-left menu bar and then choosing Sleep. Wake the computer after it has gone to sleep.
- Restart your Mac by choosing the Apple () menu from the upper-left menu bar and then choosing Restart.
- Shut down your Mac by choosing the Apple () menu from the upper-left menu bar and then choosing Shut Down.
Resetting the SMC for Mac Pro, Intel-based iMac, Intel-based Mac mini, or Intel-based Xserve
- Shut down the computer.
- Unplug the computer's power cord.
- Wait fifteen seconds.
- Attach the computer's power cord.
- Wait five seconds, then press the power button to turn on the computer.