Reset from RouterOS
If you still have access to your router and want to recover its default configuration, then you can run command "/system reset-configuration" from command line interface or do it from System -> Reset Configuration menu in graphical user interface.
Most RouterBOARD devices are fitted with a reset button.
Using: unplug the device from power, press and hold the button right after applying power and hold the button for 5 seconds (USER LED will start flashing). Now release the button to clear configuration.
Note: If you wait until LED stops flashing, and only then release the button - this will instead launch Netinstall mode, to reinstall RouterOS.
Note: More reset button functions can be found in https://wiki.mikrotik.com/wiki/Manual:Reset_button
Jumper hole reset
All RouterBOARD current models are also fitted with a reset jumper hole. Some devices might need opening of the enclosure, RB750/RB951/RB751 have the jumper hole under one of the rubber feet of the enclosure.
Using: Close the jumper with a metal screwdriver, and boot the board until the configuration is cleared.
Jumper reset for older models
The below image shows the location of the Reset Jumper on older RouterBOARDs like RB133C:
Note: Don't forget to remove the jumper after configuration has been reset, or it will be reset every time you reboot.