maybe you can take a small pin, put a really tiny dab of CA glue on it, put it in the key hole so it touches the stuck part of the key. Wait to dry and then pull the pin out hopefully pulling the key out too.
If that does not work, you can always use a hacksaw or an angle grinder(~$10 at Harbor Freight Tools).
I bought this angle grinder a while a go and it easily cut an old U lock I had:
http://www.harborfreight.com/4-1-2-h...der-95578.html