key will not turn in lock

I have a 2001 isuzu rodeo. The key will not turn in the lock. It would do this ever now and then, and I would be able to turn the steering wheel a little bit and eventually it would turn. I went to the grocery store yesterday and I tried for hours to get the key to turn. All the tricks would not work. What is the problem? Is it the tumbler in the lock or something else? Thanks for all help given.