[Daves55Sedan]

Put the key in and pull upward on the fob and turn left at the same time. It might turn that way. If so, two possibilities: the light gage metal keeper at the top of the tumbler cylinder may have worked loose, or the key itself is so worn that you may need to have a new key undercut so the tumblers will clear the keyhole.
In order to inspect the lock cylinder, remove the ignition switch (disconnect battery first). Put key in lock cylinder and turn until the button on top near the back falls, then pull out by the key fob. Be careful to observe the keeper on top as you pull out. If it is loose, the little springs inside the tumblers will pop out and you may loose some.
Prior to re-assembly, add a drop of 3-in-1 oil into the lock cylinder while the cylinder is upside down so the oil will flow into the tumblers. Then get a tube of lubriplate and smudge a bit of it on the backside of the lock where it engages with the switch actuator.