Dont worry too much.. I too have the same issue.. I have always been real thin but then i got married and lost my sick pack abs.. and i never exercise i eat what i used to eat all the time but since i have gotten older my matabolism has slowed alot and i am not active either so that doesnt help.. i seem to have lost the urge to be and stay in shape and pretty.. maybe cuz i dont need to impress anyone but it still sucks.. The best way i can tell u to get it back is to be dedicated.. do sit ups every day at night and in the morning. Do pilates twice a week.. that will do it too.. but as for me.. i just lack the motivation..
good luck..
\"Dance like no one is watching, Love like you will never be hurt, Sing like no one is listening, Live like heaven on earth!\" Mark Twain