You need physical direct access.
1) This varies by server model but the general process is to power on or reboot the server and press F2 until you get into the BIOS/SETUP.
2) Navigate to the iDRAC Settings -
3) At this point you can either default everything which will restore the default root/calvin login information or you can go to User Settings and change the username and password.
NOTE: Ensure that the user is set to Enabled and not Disabled.
4) Save the settings, and click the Finish Button in iDRAC config; and then click on Exit to leave the BIOS and restart.
NOTE: After this the new password should be active and enabled.
You still cannot login to iDRAC.
Make sure you did Step #4 above
RAC0212: Login failed. Verify that username and password is correct.
NOTE: If in doubt just reset to defaults and be sure the network settings are as you want them (eg. DHCP or Static).