Once your lender has repossessed your property they will take steps to sell your house to settle sums owed. They have an obligation to try and sell the property for the best price that can be reasonably be achieved, however this is very often less than you would achieve if you were to sell on the open market.
As such this can mean that the sum achieved in the sale of the property was insufficient to clear what was owed. Often the terms of the mortgage will allow for any shortfall that has arisen to be pursued. You can make a repayment offer for this or ask if they will settle in full and final settlement for less than is owed.
Sometimes the lender will agree to not pursue the shortfall when they look at your personal and financial circumstances. If your lender continues to pursue for payment and you are struggling with this you should look at your debt options.