Go to the Diagnostic page. Are there additional alarms on that Page along with alarm 28.
Also what type of control do you have?
Normally there will be other alarms. What are the status of the Servo drives and spindle controllers?
Did you check all thermal relays to see if any have trip? All these questions are important in order to determine any and all problems you may have .
Check status of other alarms first.
Does this alarm occur at Power Up?
See if you can answer the questions and post back with your findings and I am sure someone on this site will be able to help you out more.