Let the watchdog conter overflows, so your program will be interrupted and will restart from beginning. Anyway, the question itself do not make sense at all, considering that this would imply in a break on the program execution.
Set a flag in the ISR which is tested at an appropriate point in the while loop and causes it to execute a 'continue' statement.
At least that way you retain control of whatever processing is occurring in the 'while' loop.
Susan