The only way to recover a unit that doesn’t boot up properly is by using the debug serial port.

If it is only ethernet access that is not working, then you can also use the USB device port to log in as BT mentioned.

If you are developing for the OCG, it’s generally a good idea to have access to the debug port, especially if you are making any boot or kernel changes. Once you have working firmware that is reliable, then the debug port shouldn’t be as necessary.