This is a pretty basic one, but I don’t do it enough (or maybe I’m doing it just enough) to remember sometimes.
This works in RHEL/Centos, should work in just about everything else too.
When grub loads up at boot, hit ESC to stop the countdown timer. Now hit “a” to append a line to the boot string. Last, type ” single”, yes that’s SPACE SINGLE, and hit enter. The box will boot without loading any services and dump you to a shell. When you are done, type exit and it should continue booting.