Key insertion is one of the most basic function for RWD cars as such I don't know of any anti-theft mechanism installed. There must be mechanical issue regarding this (apart from using wrong key).

Couldn't insert the key at all or halfway?

Try to use the key at other locks eg. trunk, door. If the key is ok with other locks then the problem lies with the ignition tumbler itself. There's a protective flap closing the ignition keyhole. There could be dirt/foreign body preventing it to open. Try to use solvent such as carb cleaner to clean the tumbler.

If you live in cold climate the tumbler could be frozen. Try heating it with hairdryer. If all else fails you might need to call the locksmith.

Amarin.