There are several things that can go wrong with a lock and a key simply not turning is one of them.
A broken barrel, gearbox, or lock body are just some of the reasons why keys stop turning. Other reasons include seized lock pins, faulty levers, and a broken bolt thrower. Many of these problems are lock brand specific.
It might be possible for you to fix the problem yourself or you may need to call a qualified locksmith.
The DIY Route
Over time, dirt and dust can affect a lock cylinder, particularly on outside doors. Once the pins get stuck in a raised position, it prevents the plug from turning. Fortunately, this problem is easy enough to fix.
Spray some dry lubricant in the lock to loosen the plug and open the door. You should also spray lubricant in the keyway and insert the key to work it into the mechanism.
Calling a Locksmith
If after adding the lubricant, you can only insert your key partially, there may be a different obstruction affecting the lock. This is when you want to call a locksmith. Trying to remove an obstruction yourself could result in more damage.
It may not even be an obstruction at all. Sometimes an old cylinder or worn-out or corroded pins are the issue, which can only be fixed by a locksmith.