This has happened to me twice during my life, and both times I was unable to remove the key from the lock without it snapping and being trapped inside! I then had to contact a locksmith as I was unable to enter the property.
I can't remember who I used the first time, but the last time I had this happen to me I used a company called Evander, who were quite reasonable price wise considering they arrived afterhours (www.evander.com/), but there are many other 24-hour locksmiths available should the same thing happen to you.
Thankfully a quick search on Google via your mobile phone will bring up local 24-hour locksmiths should the same thing happen to you :o)
When I saw the locksmith remove the key from the locked was he actually took the lock off the door so he can get to the cylinder. Once he got the cylinder we were able to see the other side of the lock and was about to pull it out.