Atmel SAMD21 microcontroller not making a USB port anymore

My custom board with Atmel SAMD21 microcontroller (same as Arduino MKR Zero) is not making a USB port since yesterday. The board was working fine before that.

Please suggest ways to resolve this issue. What are the possible reasons for such an issue?

Since reset is equivalent to powering the board ON and OFF which has not resolved the issue, can burning the bootloader again help?

Further information:

1. The board seems fine. It powers ON and OFF in the usual manner.
2. This problem with the USB port connection happened while I was trying to upload the code.
3. The USB pin connections on the board have been checked as well as tested with different USB wires.

Appreciate your help.

Solutions to the problem:

1. External Hardware Reset -- using a reset button (that I externally managed to solder) -- to get into bootloader mode and start the program from the beginning

2. Burning bootloader again may also work (theoretically) - I have not tried and tested it.

For detailed understanding - follow this post - https://forum.arduino.cc/t/atmel-samd21-microcontroller-not-making-a-usb-port-anymore/882504/5