Restart the switch, and break into ROMMON using Ctrl + Break sequence. If you are using hyperterminal, ctrl CCC else refer to the following page
http://www.cisco.com/warp/public/701/61.html
Once you are in rommon, issue a "dir bootflash:" and then boot off using the valid image in bootflash using "boot bootflash:"
If you are booting from a flash card use slot0: instead of bootflash:
If you do not have a valid image, the following page should help you in recovering the Sup
http://www.cisco.com/warp/public/473/26.html