I can no longer see pictures in a folder in a grid, only one at a time

That IS the grid view that you are seeing, but you have increased the thumbnail size to the max. As ​@KR Seals has said, press “T” to get the toolbar back, which includes the thumbnail size slider, then use that to reduce the thumbnail size to something a little smaller.

Your tool bar with the view icons is hidden. 

Press T to get it back. Also, the letters G, E, D, F and a few more work fine if the Tool Bar is hidden like your example.