Hi,
This is a very open scenario, we need to narrow down to get to the root cause.
Both the sup should have identical IOS on it if thats not the case then we might run into such scenario.
try to console into the sup that is rebooting and use ctrl + break
this will bring the sup to rommon mode
then manually boot it with the IOS used on the working sup.
This should fix the problem.
Let me know whats the status.
regards,
Varun