My phone is not booting

for the first time ever, my phone started lagging so badly that i had to restart it.

when i restarted it, it doesnt boot past the 'vivo' welcome display.

it turns on, displays 'vivo'then turns off on its own. automatically turns on again displays 'vivo'and turns off. it repeats this cycle until my battery runs down before it offs completely. I took the phone for repairs, suprisingly the battery, powewr flex and charging port are all working properly but the phone cant be fixed.

please any way out?