I think something might be preventing the reverse gear from engaging. Try shifting to first gear and then back to neutral, all while holding the clutch in. Is it not even going into the reverse slot? Like, is it getting stuck in neutral, or is it just not engaging? You could try gently rocking the car back and forth while it's in neutral. Or, try shifting into first gear, rocking the car slightly with the clutch in, and then shifting back to neutral. Then try going into reverse again. Or, as a last resort, try turning the car off, shifting into reverse without the clutch engaged, and then starting the car while it's already in reverse.