Why do I get a vertical grid in a new scene?

I am new to Unity so I don’t know all the ins and outs yet.
I created a new 3D project, and the gray grid lines that normally represent my ground plane are now vertical. I’m in perspective view and Y is my up axis.
Why is this and how do I reset it back to normal?

Well, nobody has answered why this happens, but at least I figured out how to get rid of the vertical grid. Click on the 2D button to bring your viewport into 2D view, and then again to go back into 3D. The viewport refreshes itself and the vertical grid is gone! Must be just a small viewport bug in Unity.

Got this issue in 2019.1.6f1.

I was able to fix it by closing the Scene View (right click on its tab and Close Tab) and opening it again.

Must be a bug in the Editor, I had the Y grid as well, restart the Editor solved the problem, 2018.3.12f1

This didn’t work for me (2D toggle thing). My grid is in parallel to the camera direction and vertical. It’s hard to see anything properly.

Going to 2D and back to 3D did it for me. v2018.3.6f

None of those options worked for me, but instead I pressed f to focus on the camera and that fixed everything. (Windows 10)

New to Unity, stumbled across this question looking for an answer.

The solution for my issue was to select a different option in the Grid Axis dropdown (accessible via the Scene toolbar options (see attached):

Thought I had the same problem but it was actually the probuilder grid that I needed to toggle off.