For some reason I can no longer advance the selected image from the film strip by pressing the left or right arrows. I thought I had a defective keyboard (after a small coffee spill) so I replaced the keyboard but to no avail. Hopefully someone can help me resolve. The up and down arrows do work in the G view mode but no lateral movement with the L R arrows.
Assuming you are talking about Lightroom Classic.
It's working fine for me in LrC 11.2 and Win 10.
Are the left and right arrows failing in the Library module or Develop module? It is possible for arrow keys to not affect the filmstrip/grid if keyboard focus is on a field or control, because then the arrow keys adjust the control. Whenever that happens, you can override that by adding the Command key in macOS (Ctrl key in Windows). So try pressing Command-left arrow or Command-right arrow.
If absolutely nothing in the current module has keyboard focus, the left arrow or right arrow alone should be able to move through the filmstrip. One way to try and escape all focus modes is to press the Esc key until the left arrow and right arrow work in the filmstrip.