Is it a wireless mouse, if so check the batteries.
If it is chorded try another mouse and see if the problems persists.
Check these settings:
Go to the Control Panel and select Mouse. From the mouse properties screen click the Hardware tab. on the next screen under the Genral tab click on Change Settings. Then select the Power Management tab and make sure it is set to "Allow this device to wake the computer" and not "Allow the computer to turn off this device to save power"
This is correct instruction for Windows 7, if you have a different OS (which you should have stated) there may be some minor differences.