One other possible problem is foreign matter in the keyhole. If none of the other suggestions work, you might want to try blowing it out with compressed air. This is usually only problem for people with small children :-)
Sometimes rotating the steering wheel slightly back and forth while wiggling the key will make it turn. You could also try a different key if you have spares. It is improbable that any corrosion would form inside the ignition key cylinder, but places like NAPA make specific lock lubricants like this with graphite powder suspended in them to help free the pins in stubborn locks.
Spray some WD40 in it . insert key few times . then take small hammer and tap key several times while wiggling . This will usually get it to turn . Then you need to replace lock cylinder before it does it to you again .
