I have had this problem several times. Sometimes it solves itself by rebooting. I am running 10.4. It seems related to updates but can't be sure. My task bars are missing and when i minimize the screen disappears.

I was going to reinstall the program and when I went to reinstall I realized I had forgotten my password so when I tried to go through the grub command on the start up I saw that grub was no longer on the screen only two error messages. No safe mode found and Unknown command terminal.

so four issues: task bars/password/error messages/no grub command