2001 Kia Sephia Doors won't unlock

I have a Kia Sephia and the doors won't unlock. This happen once before and we were inside the car and couldn't get out because the lock/unlock button wouldn't move to unlock, it was stuck on both the driver side and passenger side. Eventually it finally just unlocked after we kept messing with it. A couple days later, I unlocked my Kia with the key, drove 5 minutes to the gas station, locked it from the inside before I shut the door, came back out and it won't unlock on either side. I had a tow truck come and even he couldn't unlock it with the tool. He had to unlock the back door (doesn't have a key spot) and we have to climb through the back. What is the problem? How can both doors break at the same time?