The official documentation for this procedure is available at Recover Password for Catalyst Fixed Configuration Switches. What follows below is a condensed version, for our mutual convenience:

Hold down the mode switch while power cycling the unit. Issue the following over the serial terminal (9600 8 N 1) and note that load_helper may not be available on your device:
flash_init load_helper rename flash:config.text flash:config.bak boot

Abort the configuration wizard by specifying n at the prompt: --- System Configuration Dialog --- At any point you can enter a question mark '?' for help. Use ctrl-c to abort configuration dialog at any prompt. Default settings are in square brackets '[]'. Continue with configuration dialog? [yes/no]: n

Enter enable mode at the Switch> prompt:
en !-- Recover the old configuration thus: rename flash:config.bak flash:config.text copy flash:config.text system:running-config !-- Then commit new secrets to the configuration: configure terminal enable secret password enable password password line vty 0 15 password password login line con 0 password password write memory