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I don't know what causes it but it's often the worldspawn classname that's got deleted, so all the brushes will disappear.
It can usually be fixed by opening the .map in notepad and adding the line "classname" "worldspawn" (with quote marks) below the first {.

If you've saved it and the file size has gone right down, it might be too late to fix it unfortunately.

It sounds like a badly made brush may be causing the problem.

Save the .bak version as <mapname>_backup.map and then save it again as <mapname>.map
Now open mapname.map and slowly start deleting things and trying to compile, keep doing so until the error is gone.

Tassie wrote:Drofder's way worked, woot. only had to delete half a room :S lol but atleast it compiles now, cheers.

Yeh, now you know what the problem is, open up the backup version, and delete that brush...

Then you delete 1 brush instead of entire room