Are you using any type of telnet connection? I see that you say that the configs aren't different. That informs me that you were able to see the CLI at one time. Are you able to see the CLI when connecting directly to the console port from your PC?
It could also be the baud rate of the telnet program you are using. Make sure you are using the default baud rate for the Sup model you are using.
Also are you using the same Sup's? Once you push in the sups, one will be the active and one will be standby. If you are connected to the standby sup you will not be able to type any commands, but it will inform you in the hostname that it is the standby sup.
Hope this helps.