Chosen Solution
Hello Today I tried to update my macos mojave and midway update it restarted 2 times, second one it started beeping and black screen. It fan was spinning fast when it was installing. Now nothing works, tried every command and apple support could not help. Ram was never touched and never been water damaged or dropped.
A beep every 5 seconds is provided when no RAM is found by the system. Since on your Mac the RAM is soldered to the logic board, that’s clearly a board issue of some kind. Usually a faulty chip causes integrity alert with the 3 beeps sequence, thus it might be a component carrying power or data to the RAM chips got damaged. In my opinion it would be worth having it looked up by a lab into component level repairs.
Try booting from an external USB drive and run Disk Utilities on the internal drive. An alternative would be to put the internal SSD drive into a OWC Envoy enclosure and repair it using another Mac. Get a good system on it then try booting from it externally before moving it internally.
