insurance co. denies claim following pre-auth

sacto,ca. obtained a pre-authorization for needed major dental work. benifit amount is 1,550.00. was uncertain of exact coverage ending date since domestic my domestic partnership had just ended and knew it would soon be cancelling. explained all details and concerns to ins.co and was informed that coverage to be effective until last day of April, they then issued, in writing a "pre-authorization" to my dentist for 1550.00. again had dental hygenist verify coverge on day treated April 27. again confirmed coverage. Now denies coverage, claims employer had "backdated" my benifit date to march 31st, after the fact. Employer states "did not backdate", just "updated files on may 17th. neither ins co nor emplyer will take responsibility for missinformation, both are blaming other for the error. I am being stuck with the entire 3,500.00 bridgework bill at this point. im already broke and part.disabled havent the ability to pay. am wearing a temp.device covering the three, now missing front teeth. this temp will remain until bill is paid hopefully before temp wears out. i am so stressed out about this. please! help me if you can. thanks