You need to escape the first {
See this example: http://bigfix.me/fixlet/details/6072
It should be like this:
waithidden MsiExec.exe /X {{90120000-002A-0000-1000-0000000FF1CE} /qn
I’m not sure why it would become unusable after because it shouldn’t have done anything at all. Is this the correct GUID to uninstall?