ignition lock cylinder stuck

I have a 91 calais Q4 newly rebuilt that has had no serious problems till now.Went to the bank yesterday and when i returned i couldnt shut it off ,the key is stuck in running postion and i can not take it out.I had to disconnect the battery just to shut it down.Has anyone had this problem? Could it be electrical? or do i just need to replace the lock cylinder? I havent had any problem with the lock before it just jamed all of a sudden and locks rarely go bad all at once.Where is the key release switch when its in park? Doesnt mention it in the haynes manuel and im afraid som1 will see the key and try to break in.Many thx to anyone who can help.

I had that trouble once before and replaced the ignition switch. But today after about a year with no problems, all of a sudden it did it again on my 1985 Cutlass supreme.