1. You need to describe your problem in greater detail. "Stuff is disappearing" doesn't give us a good clue of what's happening. Stuff is being removed on compile? Things are randomly disappearing as you run around the map? Hammer is randomly deleting stuff? We can't read your mind.
2. If it's a problem that appears after compiling, always check your compile log, TF2, or the Alt-P problem checker for errors.
3. If you're totally clueless, just post your .vmf and perhaps someone else can figure out what's going on. This should probably be a last resort.