Installing random debs off the Internet isn’t a great idea unless you take careful note of what is being installed, how it works and importantly understanding any potential conflicts and packages that may be removed.
You can try this After update last night computer doesn't start anymore - #2 by fossfreedom
If that doesn’t work, use whatever instructions the postmaster project says to uninstall.
If that doesn’t work, basically reinstall is the quickest route to get back to a working desktop.