Is the fast of one who intended to fast on Thursday valid, then woke up after the time of Fajr began and drank and ate (as suhoor) and did not see the break of dawn, so should he complete his fast or break it?

If a fasting person eats or drinks after dawn has appeared, their fast is invalidated, based on the words of Allah the Almighty: "And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the night."