What steps should I take to fix my desktop not working?

This is just a suggestion for a try out:

Boot from an Ubuntu Install ISO.

If successful, chroot into the partition where Ubuntu is installed (Check internet for guides for your type of installation).

Remove ubuntu-desktop package from terminal and re-install the package.

Please note that, this may not be possible in chroot, but worth a try.

Even if removing that package is possible, you should be able to reach a terminal.

If it is just installed yesterday, if nothing else helps, re-installing doesn't seem to be a big inconvenience unless you worked on the installed system for many hours.