Is a Commitment Letter a legal binding document?

I received a commitment letter from the U/W dept. from my lender saying my mortgage application was approved!! I call the loan officer who I have been working with and he said he has not received anything yet. He then calls me back and tells me that I received that letter in error and the application was not approved!?My Question is can I fight this or I can't do anything because the commitment letter is not a contract? I have a signed commitment letter from a mortgage specialist from the U/W dept.