I know this is an old topic, however it comes up in google searches when people go searching for solutions to Golf MK5 door lock issues.
I thought I'd share some of my findings as it might help others searching for a way to the dreaded door hard locked issue solved.
I had this issue the other day, my mk5 rear door locked itself solid, you could hear a clunk-da-clunk when I pressed the button - but the door would not unlock and could not be opened from the inside either. I was getting rather concerned that an angle grinder might be involved in getting it open! I have pulled these doors apart and repaired the locks before - it's not hard at all - however if you can't get the door open - you have a bit of an issue fixing it!
When the car engine is off, all you have to open the door is residual battery voltage, when the engine is running, you have a fair bit more voltage because the alternator is charging the battery - this is what got my door open, the fob would't open the door while the engine was off, so I started the engine, revved a little and pressed the internal unlock button on the driver's door - lucky for me this worked!
The problem is that the mechanisms are run by tiny direct current motors. Over time, the motor brushes gum up the contacts inside the motor - meaning that normal voltage might not be enough to get the motor to turn - add a little more voltage and it should be good to go. Once you get the door unlocked, you'll probably find it will keep running ok - but DON'T TRUST IT! As soon as you can, either get a pro to replace the lock mech for you - or look up how to pull the door apart and get to the lock yourself. When I get on a computer next, I'm going to upload some pictures of how you can REPAIR the door lock yourself for little to no cost because these things aren't cheap - and most of the time, there is very little wrong with them - usually nothing a $5 motor from ebay won't solve! In all cases with mine, the small motor contacts were the issue, others have had problems with the micro-switches on the circuit boards - these cases are a little harder to deal with, but I have seen people fixing them before too.