With the breaker off, take the outlet apart and check the wires. The black should go to the gold screw and the white should go to the silver screw. If not, they need to be switched. This is a simple fix. If they are correct, the wire is reversed at the breaker box. Or there may be another outlet in the line somewhere that feeds the outside service that could be reversed. If your not familiar with your service box you should hire someone to take care of this. If you don't already have one, the outside outlet should be GFCI. This would be a good time to replace it.