Welcome to Android Central! Is Google Play even opening, or is it crashing before you even see the main page? Or can you open it, but can't install anything?
If you can open Google Play, tap menu/Settings/Account, and make sure your Google account is selected. If you can't open Google Play, try going to Settings/Apps/All, selecting Google Play Store, and Clearing Data and then Force Stopping. Then try opening Google Play again.
I noticed that Google Play Services updated today, so I wonder if things might have been a little funny while it was rolling out today. The current Google Play Services version I have is 3.2.25 (761454-34).