How i can reset the password in a enterasys C3G124-48, if lose it

Before trying anything too drastic, I'd try using the username "admin" without the quotes and leaving the password blank. If that doesn't work, then you'll have to use the reset switch that's located on the back of the switch (number 1 in the pic below).

The following steps are taken from the manual for the switch.
1. Press-and-hold the reset switch while the switch device is operational. This change of the login password to the default password will be indicated through CLI only.
2. Logon the device management. You can now logon to the switch through the console port and assign a new password using the command line interface (CLI).