An encoder problem on one axis may prevent homing, but it will not prevent you from jogging the other axes. There is something else going on.
The status message window in the upper right corner of the screen is your friend.
Choose Slow, Incremental jog mode and press variously the X, Y and Z jog buttons. What messages appear in the status window? Do any of the axes move in either direction? Switch to Slow, Continuous jog mode and try each direction again. What messages appear in the status window this time?
Before you try to home, make sure all three axes are well clear of their limit switches and hard stops. Power off and turn the pulleys by hand if you need to.
When you try to home, and it says "411 Z axis(3) full power without motion", does the Z axis actually move (e.g. run away rapidly)? Or does it sit still? Or does it move under control at the usual slow jog speed until it reaches the switch or the end of travel, then give you the stall message?
Assuming that none of the axes move at all when you try to jog them or home them, then you probably have a servo drive power problem. Perhaps you are not getting DC power to the VM terminals of your servo drive. Perhaps the limit switch cable between the RTK2 and the servo drive is loose (assuming this is a pre-2002 M400, not a newer M400S model).
Is your control a pre-2002 model with a SERVO1 drive; a 2002-2004 M400S with a SERVO3IO drive; or a post-2004 M400S with a DC3IO drive? See Centroid Servo Drives and Field Guide to Centroid Controls, 1994-2010 - CNC Services Northwest for pictures if necessary.
What software version do you have? The software version number is displayed on the F1/Setup screen.