The key is hopefully in the right position to set the pins to the break point. If it is then all you need to do is turn the tumbler by inserting a small screw driver in it and twisting.
If you are not so lucky try pushing the key in a bit further....CAREFULLY and slowly...bit by bit, and keep trying the screwdriver. If you can position whats left of the key it should open the lock.
Failing that you have two options. Drill from the back and push it out, or drill from the front and take out the tumbler completely, or at least enough of it to remove the pins and then turn the tumbler with a tapered bolt or easy out.
Usually when I run into this I just drill the tumbler with a very sharp drill about 2 sizes smaller than the tumbler. I can then pick out the lock pins and turn the lock with an ez out.
Of course you then have to replace the tumbler and pins, but its cheaper than a locksmith and cheaper than a whole new lock. When I worked as a maintenance man for a 300 room motel in northern California I had a complete lock pin set and plenty of spare tumblers. On average some idiot would deliberatly stuff something into a lock and Id have to drill it just to let the maids into the room. Got to where I could do one in about 20 minuets. Rekeyed and all.
Please let me know if you have any other immediate concerns.
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