Wait a second, they claim you have not paid for 2 yrs, how do they know you have not paid, it was cash, as such if the person stole the cash how do they know it was your cash and not another persons cash. Turn the tables on them, have them prove you never paid. Have them prove you have been using the gym for the last two years without paying. At this point they are assuming you did not pay. Ask them if you did in fact not pay, how come you have a valid member ship, just because they have no record and have poor record keeping practices is not proof you never paid.
When thing like this happen I try to turn it around on them.