key broke and stuck in U-lock with bike locked
If the key's broken flush and you can't grab it with a pair of needlenose pliers, you should call a locksmith or the police. 

In all likelihood, they will just cut the lock with an angle grinder or a hacksaw with a diamond blade. 

Though from the picture, it looks like the lock mostly is open and maybe a good hit with a hammer might dislodge the lock.