Happened a lot to me whilst I was missioning to level up.
I found that sometimes, if it happened before you get into the mission area, you could run and jump at the invisible doors and you can get through into the area but when inside, or if it happens whilst you are inside, there is no way to get out unless you /suicide.
Bit of a bugger if it happens at the end of a long series of mission points and you can't complete the mission and you have to restart. Luckily if you do get inside but can't get out using /suicide doesn't end the mission. Of course if you have to escort an NPC somewhere its generally a diferent story...
