Read the firmware change plan before you update or switch firmware. It covers what to back up and how to roll back, which makes every later step on this page less stressful.
If something has already gone wrong, work from the least to the most drastic fix: first the admin-panel-not-loading guide, then the factory reset and recovery guide. If the hardware itself seems faulty, the return, refund and warranty page explains the vendor's published terms, and the safety page summarises the company's documented security record.