The 3550 Catalyst switch does not have IOS recovery functions other than xmodem, when the IOS is lost due to corruption or maybe user error. You have to copy it with xmodem back to the switch. I would suggest, taking a smaller image first, because this take quite a long time. For an 3,5 MB sized image, it would take 1 hour and 10 Minutes.
Boot the switch with the missing IOS. Start the command:
switch: copy xmodem: flash:new-filename.bin
And then start Hyperterminal and send the binary file from your local harddisc via xmodem.
Source:
http://www.cisco.com/en/US/products/hw/switches/ps628/products_tech_note09186a0080169696.shtml