1) Make sure that your iPod meets the requirements for the OS as dictated by the app. 2) Try restarting the iPod by turning it completely off and then back on, some apps require that before they can be used properly. 3) Re-sync your iTunes stuff onto the iPod touch, or download a free app directly onto it, your iPod could have forgotten that it should be authorized to run your apps.
If all else fails THEN restore.