What if the person who I got my house from didn’t tell me about the back taxes that are owed?

If they gifted it to you, they had no obligation to tell you about the taxes: unlike when you buy a house, where because you are paying value ("consideration") for the home, the seller has a legal obligation to disclose issues (legal or physical), when you accept a gift, you accept it as is--it's the equivalent of someone giving you an older car with a shot transmission that needs a replacement costing as much as the car is worth; just as in that case, you accepted the car problems and all and can't make the giver take it back or pay for the repairs, so, too, in this case, you accepted the house with $20k in back taxes and are no stuck with it. 
For future reference, don't accept potentially costly gifts without checking them out: you could have checked for liens, back taxes, etc. before accepting the home (and therefore had the chance to reject the gift if you wanted), the same way that if someone offers you a gift car, you can insist that they let you have a mechanic examine it first, or else walk away from the gift. 
If the house is worth less than the $20k, let the town or city or county (whomever asses property taxes where you are) take it. If it's worth more, redeem the house and you still come out ahead since you did not pay for it.