Ignition key

A couple you might try that has worked for me in the past. Try spraying a little WD-40 or something equivalent right into the key cylinder. This should oil up the inside and prevent it from binding. Another thing is to make sure that the key itself isn't bent, twisted, damaged, or just worn out too much. Ive owned a vehicle where my key was slightly twisted causing me problems. I made a new key and it fixed my issue.

If these things don't work, then unfortunately, the key cylinder will need to be replaced.