The board becomes "bricked." It enters a state of permanent amnesia. It will not output a boot logo. It will not accept a signal. It becomes a piece of dead e-waste, salvageable only by desoldering the main chip and using specialized EEPROM programmers to manually inject the code via microscopic probes—a process that costs more in labor than the board itself.
Firmware updates often perform a factory reset or partition reformatting. Back up: tp.sk518d.pb802 software update