Is one who wakes up after the Fajradhan, having intended to wake up before sunrise, but then wakes up after sunrise, sinful?

If the adhan (call to prayer) you heard was the first adhan for Fajr prayer, before its time had entered, then there is no sin upon you for missing the prayer, because sleeping before the time enters is permissible. However, if the adhan was after the time for Fajr had entered, then you are sinful for neglecting the prayer until its time expired. You should take measures to wake up, such as setting an alarm, and congregational prayer is obligatory upon adult men.