There are many ways of doing it,
1. If it has a round key, take a BIC ink pen and remove the cap from the non writing end, then jam it in the key hole and twist it like a key.
2. If not a hacksaw or battery operated grinder will make quick work of it.
3. Also a car's scissor jack can force it open if you have enough space to get it in there.
4. oxy Acetylene torch also works.
5. If it's not through the frame you can remove the wheel and take the rest.
Look on youtube under U-lock picking and you'll find more videos than you can watch in a day. Lots of ways to get through it, just takes a little ingenuity.