Item can't change in Minecraft, which key must I press now?

I just stupidly pressed any key on Minecraft and can't change the stones anymore. I always stay with the gray stones, I can't change, for example, to the grass. How do I solve the problem now?

In the inventory, the slot is next to the armor, to switch to F

Press F

F ⠀⠀⠀⠀⠀