is the rest file a generic mainfirmware of k750i?
The flasher programs(XS++, SETool2 Lite ect.) need to bypass the phone security to be able to communicate with phone(to be able to flash ect.). They flash a breakin loader (to bypass the phone security), i.e. they overwrites a few blocks in start of MAIN. A rest file (restore file) is a piece of main file(4-8 blocks), and is used to restore the overwrited MAIN blocks. Instead of rest file you can reflash the entire MAIN(commonly 300+ blocks), but it will last longer. Now I hope you know what is a rest file... :)

do only I have to reflash K750_R1DB001_MAIN_EU_1_CL_RED49.rar to bring back alive my cell?
Yes, just reflash your phone with MAIN firmware.

do you think I should repair the EROM (
First reflash your phone with MAIN, and if still not work, then you can try.