The gauges on the Celica's are all electronically driven; therefore it is entirely possible that when whomever serviced your car, did somthing that interfered with the speedometer's circuit. (Ex. Short circuit, double ground, pinched/cut wire. Etc.) It's also possible that it's none of the above. One thing you might be able to try before servicing it would be to disconnect you car battery and let it sit for a few minutes to reset the entire ECU and main harness.