Many thousands of Americans who lost their homes in the housing bust, but have since begun to rebuild their finances, are suddenly facing a new foreclosure nightmare: debt collectors are chasing them down for the money they still owe by freezing their bank accounts, garnishing their wages and seizing their assets. The two big government-controlled housing finance companies, Fannie Mae and Freddie Mac, as well as other mortgage players, are increasingly pressing borrowers to pay whatever they still owe on mortgages they defaulted on years ago.