Close the app then close it from your multitasking bar. Double tap the home button and it will appear. You can then close the apps like you would delete one from your springboard...
If after closing and reopenning it still doesn't work. Try deleting it and installing again. I had that same prob before and this fixed it. You will lose all your game data unless you sync the iPhone to iTunes and load the backup files back from iTunes to your phone after deleting it.
Soft reset is what worked for me, I guess I have too many apps installed to play angry birds smoothly. In any case you can uninstall the app and reinstall it (or just play angry birds online on chrome, besides the angry birds golden eggs, it has secret dimension levels).