Sorry mlx it was just there was a thread similar a while ago and it was a double glazed door, I would have given the link for you to see if it helped. Not sure what sort of handle you had on the door but it may just have not helped to unscrew it but wouldn't know unless we could see. Hope you get it sorted, if you can't do it yourself I would definitely try to get someone in to do it rather than damage the door, my opinion anyway.
If you can remove the stop strip you could try forcing something thin into the gap and pushing the catch back enough to open the door.
I would get a professional rather than resort to violence. It could prove expensive.
See when you removed the handle, were there 4 wee metal bits around the rod type bit that goes through the door? (this is very hard to describe). Make sure they are all in place and not sliding out. It happens with my bedroom door all the time and stops the handle from turning, but now I know what causes it I can fix it in seconds.
This happened to us late one night. My son came out of his room and shut it. When he tried to go back in there was a click of something breaking inside.
Eventually I had to kick the door until it started to crack, then with a mighty shove I got the door open.
After replacing the mechanism I managed to glue the door back together again. I used the clamps from my jump leads over an old towel to hold it while it dried.
A bit of filler, sandpaper and paint, and you can hardly tell that anything happened.