Mine was broken two months ago, I took the phone to a Nokia Care Center, and they fixed it. (My phone doesn't have warranty in my country, I had to pay for the repair)
Then, the issue happened to my friend's Lumia 620. He opened his phone, and saw that the solder of the lock key is loose. He tried to fix it by soldering it. His lock screen has been fixed, and is working now. (sorry if my English is not good)
And after that, the issue happened to my phone again. I think something's wrong with my Lumia 620. While he was able to fix his phone, he was not able to fix my phone. Actually, he tried to fix my phone two times, and just after one day the issue appeared again.
I'm thinking about taking my phone to a Nokia Care Center again.