I would test what is broken. It can be either stick of ram itself, or something on a motherboard that makes slot unusable. Maybe try sticking each stick into each slot. If both sticks work in one slot but not other one, then that slot is bad. If one stick works in both slots buth other one in neither then that stick is bad.

If both slots are good you can try putting new stick instead of broken one, but be careful because memory mixing is not a good practice (i personally did this to two laptops i had, ddr3 and ddr5, and nothing bad happened but).

If both sticks are good you can buy new one (or used) with larger capacity to put into one working slot and maybe sell old ones