Update: PolyMC has been taken over by rogue actors and is not recoverable; apparently the project lead's true colors came out and the Discord has turned into a mini K*w*f*rms. DO NOT launch PolyMC, it may download and run malicious code any time it auto-updates a Minecraft instance before starting it.
Back up your local worlds and uninstall PolyMC, and don't forget to revoke access from PolyMC to your Microsoft account:
https://microsoft.com/consentThe legitimate developers are gathering under a new project. If you enjoyed PolyMC, keep an eye out for development of PlaceholderMC:
https://github.com/PlaceholderMC/PlaceholderMCIf you want to play Minecraft right now and no longer have a launcher, gdLauncher and ATLauncher are community favorites. MultiMC is not recommended for all of the reasons that led to it being forked into PolyMC in the first place.