During cruise yesterday my ATS failed, and since then I have been unable to use it. It is not a flashing "FAIL" message, like what comes on after using reverse thrust, but rather is a solid "FAIL" message. Pressing the disconnect button on the throttles do clear the message, but does not allow the ATS to be engaged. All circuit breakers are in, and I have tried resetting the auto throttle breaker multiple times. The ATS PIDs in the study menus also just say "PID RESET". I have also checked the failures menu, and saw no failures. Any ideas?