Backup of https://fatmin.com/2014/12/02/how-to-reset-cisco-catalyst-3750-back-to-factory-defaults/ by Christopher Paquin in case it ever goes dark.
Need to reset your Cisco Catalyst 3750 back to the factory default settings? Have you forgotten your password? Well you have come to the right place. Note that I am assuming that you have already established a console connection to the switch using a Cisco serial cable (rollover cable).
First you need to power down the switch. Once the switch is powered off, hold down the mode button, and power the switch on. The switch will boot up and you should see the switch prompt as shown below.
Now type flash_init. Your output should be similar to what you see below
switch: flash_init
Initializing Flash…
flashfs[0]: 547 files, 19 directories
flashfs[0]: 0 orphaned files, 0 orphaned directories
flashfs[0]: Total bytes: 32514048
flashfs[0]: Bytes used: 15487488
flashfs[0]: Bytes available: 17026560
flashfs[0]: flashfs fsck took 11 seconds.
…done Initializing Flash.
Check out the contents of flash and locate config.text and vlan.dat (if it exists).
switch: dir flash:
Directory of flash:/
2 -rwx 564 <date> vlan.dat
3 -rwx 1914 <date> private-config.text
5 drwx 192 <date> c2960-lanbasek9-mz.122-58.SE2
6 -rwx 3096 <date> multiple-fs
7 -rwx 2289 <date> config.text
Now delete the vlan.dat and config.text.
switch: del flash:config.text
Are you sure you want to delete “flash:config.text” (y/n)?y
File “flash:config.text” deleted
switch: del flash:vlan.dat
Are you sure you want to delete “flash:vlan.dat” (y/n)?y
File “flash:vlan.dat” deleted
Note that you can also just rename the config.text and vlan.dat if you are not certain that you want to delete them.
switch: rename flash:config.text flash:config.old
Now type boot, to reboot the switch. Once the switch is rebooted you will see the System Configuration Dialog, and will have the opportunity “to enter the initial configuration dialog”.