I had a problem a long time ago where I was unable to open Blizzard maps also. The best advice I can give you is to uninstall and reinstall Starcraft II if possible. When I did that, I was able to fix my problem. You could have somehow put the Starcraft II Map Editor into a bad sequence, or have a bad registry, which only reinstalling it can fix. Especially if after restarting the program, or restarting the computer does not fix it.
Basic questions to help that problem solving is below:
- Do you have any weird preferences set up in Starcraft II Map Editor that could cause the problem?
- Do you remember exactly what you were doing days before you experienced this problem? (ex did you try to save over an official blizzard map or try changing unit stats etc etc)?
- Did you install an add-on (not sure if there is even any) or any mods for it for the Editor?
- You mentioned that you 'restart' your computer.... Did it freeze the last time you used it while you were modifying something? Or do you mean you restart your computer (without having SC2EDIT open/used previously)?
- Is this done on a fresh installation aka a first time opening SC2EDIT, or have you used it consistently for x days in a row?
That's the best advice I can give you, and probably anyone that is a non-Blizzard employee.