First, turn your Roomba upside down and clean the battery contacts on the base. Unplug the docking station and clean any dirt or debris off the contacts there too. If the contacts are rusty the rust will prevent the Roomba from charging properly. Get an emery board or fine nail file (I used the cardboard type as they are more flexible) and gently sand off any rust. This may get your Roomba charging again.
As this is an earlier model Roomba the battery maybe due for replacement. Even a rechargeable battery doesn’t last forever. Later model Roomba’s have software that won’t let them take non-original parts but yours might not have this issue.