There can be various causes; did you use any 3d party programs to protect/optimise your map? Those could have done something wrong. Also, if you use custom editors, something might have gone wrong there.
If you used none of the above, the icons and models could be broken because of mistakes in editing and saving the pictures (Tga saved 24-bit instead of 32-bit removes the alpha channel which causes the icon/skin to be blank (teamcolor or black)).
If that's still not what happened, did anything go wrong when saving the map last time? like turning off your pc, crashing pc, anything along those lines?
Only thing that is left i could think of is very unlikely, a virus randomly corrupting files, but i couldn't believe one to only attack warcraft 3 maps. So that's most probably not it.
I would indeed recommend you to remove all imports from the map and reimport one or two things, test those first to see if it works, if they do, import the rest again, if they don't, try importing into a clean map and test that. That way you can see wheter it's your map that's corrupted or the editor doing something wrong. In case it's the second, reinstall warcraft 3. If it's the first, find a backup and pray it's recent.