When I play, I can't see my hotbar or my arm

I expected to see my hotbar to see the items I have, and see my arm as well as the item I'm holding.

What actually happened was: my hotbar is gone, I can't see my items unless I open my inventory, and my arm is gone

I don't know how to reproduce it because it happened without a trigger. I might have accidentally pressed a button, but I can't be sure