If it's possible at first reset the Bluetti. Figure out how to do this, most you must press some buttons at the same time for few seconds. Do this not work, contact Max Oak.
The last try is to open the Bluetti, disconnect the battery, wait 60 seconds, switch the Bluetti on (to discharge some condensators) and reconnect the battery. Be very careful if you do this! If this don't solve the problem, I think the Bluetti is dead.