OnePlus India — Lost all my data phone auto reset

Upon getting up in the morning, my one plus showed 1% battery. I immediately moved to charge it. It got off. Upon putting charging cable, charging plug sign with mobile charging input sign was shown (doesn’t show usually). I waited. After around 15 minutes, normal charging sign was there. However, I let the phone to be on charging for some time. Upon switching the phone on, something in Chinese language (not comprehensible) was there on screen for 4-5 minutes. Then, it started with basic information like setting up WiFi. It all happened at a time when I was in a hurry and rushed to attend my duty.
When phone started after going through all steps of setting up (at a time when attending duty and making important calls was the top priority), I found all my apps and phone data was lost. Basic essential information like contact number of my driver, attendant etc. were all lost. It has caused undue harassment to me.