I thought I used a wrong file for the upgrade which set the phone to death and I have mtk backup file from hovatek that brings the phone alive but powering it,it keep on restarting on the Infinix logo holding the power key for some seconds powers off the phone.
This is where I was confused of what's wrong because sp flash tool completes the flash.
So,I have dead phone now lead me through this please ....
Kudos
Alhadulillah,
On successful flash of my X551 today after many tries experiencing USB NOT RECOGNIZE .
I dissembled it to disconnect the battery connector and connect it back.with this I got the preloader back and sp flash tool was able to detect phone and suceeded in flashing it.