Sorry to hear about the trouble. If I get it, you are unable to use Command + to zoom in on an artboard? If yes, this generally happens when there is a conflict with any other application. Please close XD and restart your computer and after rebooting please launch only Adobe XD and check if you are able to use the shortcut. If you are still unable to use, please share the following details:
The version of Adobe XD.
Is it happening on a specific file or multiple files?
Please check if you are able to use any other shortcut which includes the Command key.