1. Try resetting your iPod to start it off. Press and hold both the SLEEP and the HOME buttons for about 10 seconds. The "Slide To Power Off" option will appear, but keep holding the buttons.
2. If the apps still won't work, try ACTUALLY turning off the iPod by holding the Sleep button for about 5 seconds until prompted to "Slide To Power Off" your iPod. I've actually managed to get some things to work after restarting it, rather than resetting it.
3. If those two are unsuccessful, re-sync all your apps (although you might lose the data you have for those apps)
4. And you last resort: restore it via iTunes.