go ahead an back it up. NOW!

The second restart sorted out the booting problem, it probably took a while longer than usual to boot, so just move on with your life and your Mac while ensuring that you backup on a regular basis.

You didn't specify what model of Mac you have but whether laptop or desktop makes no matter, they both have Time Machine which you do not have to "remember" to use. Just keep the external plugged in and you are golden

P