#clear current config on running cisco ios device
clear config all
write erase
reload
System configuration has been modified. Save? [yes/no]: n    

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Power the switch and bring it to the switch: prompt:

For 2900XL, 3500XL, 2940, 2950, 2960, 2970, 3550, 3560, and 3750 series switches, do this:

Hold down the mode button located on the left side of the front panel, while you reconnect the power cable to the switch.

Catalyst Switch Series|LED Behavior and Mode Button Release Action
2900XL, 3500XL, 3550  |Release the Mode button when the LED above Port1x goes out.
2940, 2950            |Release the Mode button after approximately 5 seconds when the  
                      |Status (STAT) LED goes out. When you release the Mode button, the SYST LED blinks amber.

http://www.cisco.com/en/US/products/hw/switches/ps628/products_password_recovery09186a0080094184.shtml