Has autopilot ever stopped on you?

Hello,I was doing a little flight with the 737-7 and was going to have a FL280...well upon reaching FL24000 the LNAV mode never would work...I could turn it on for about 1 sec. then the light would go out and heading mode would turn on...then the whole a/p itself went out and in the FMC I recieved messages of how each thing..ex. heading control, VNAV, LANV, etc...was going out and inopporable..then the fuel went crazy..going upto over 400,000..I take it I programmed something wrong in the FMC...does anyone know what it could have been?ThanksChad Moore