Can you reboot the device to safe mode by long press the power key and hold the power off to accept the safe mode and then go to the settings>system>about device
if that works then better do the reset and hope it may fix the issue
@user1520881638007 as much as I remember, the About Phone entry in Android 10 is directly under Settings and not under the Settings => System sub-menu. So, instead of tapping "System", he needs to tap "About Phone".
Answers
Enable it as you did earlier. Go to Settings => About Phone and press 7 times consecutively on "Build Number".
@singhnsk
but there(above screen shot) about phone is not even visible ,then how come he can enable the developer mode
@RINIL M
Can you reboot the device to safe mode by long press the power key and hold the power off to accept the safe mode and then go to the settings>system>about device
if that works then better do the reset and hope it may fix the issue
@user1520881638007 as much as I remember, the About Phone entry in Android 10 is directly under Settings and not under the Settings => System sub-menu. So, instead of tapping "System", he needs to tap "About Phone".