It sounds like you're operating in TMCC mode. As Richie mentioned above, make sure the "figure" switch is moved forward for "run" mode.
If your locomotive won't reverse, but all other commands (Forward motion, rear coupler, missing firing) work OK, then you may have a board problem.
But first, try to reset/restore your locomotive using the procedure found on Page 13 of your owner's manual: https://www.lionelsupport.com/...ents/72-8411-250.pdf
NOTE that the manual has an incorrect code: instead of 8, use a 5 if you'd like to have the red rooftop light flash, or a 6 if you'd like it to be on all the time.
If this still hasn't fixed the issue, then chances are you have a problem with your R2LC or ACDR board found inside.