Do you have the brushed DC servos or the Clear Path servos?
If you have the older Gecko drive DC servos I bet you poped a driver. I have 5 extra here if you want a spare I can give you a good price. You could have lost a encoder but its less common.
You can test the encoder by disconnecting the 70V line to the driver and power the drive +5V. If you rotate the servo and the lights give a yellow then red after a small amount of rotation then the encoder is working. Do it in both directions. If the encoder test good its the driver. Gecko has the test procedures at there site.
Always a chance with brushed servos you have a burnt brush and armature.