What should one do if he wakes up late for sehri [pre- dawn meal]?

The Holy Prophet ^sas said:

When any of you hears the summons to prayer while he has a vessel in his hand, he should not lay it down till he fulfills his need. 

(Sunan Abi Dawud, Book 13, Hadith 2,343).

If someone is still eating when the time of sehri ends, he can quickly finish what he is already eating. However, if the time of sehri has ended and someone has not started eating yet, it is not appropriate to start eating sehri.

Hadrat Musleh Mau‘ud ^ra was asked about a person who did not wake up in time for sehri and did not fast that day because of missing sehri. He ^ra said:

If he cannot keep a twenty-four hour fast, then he can make up the fast after Ramadan.

(Fatawa Hadrat Musleh Mau‘ud, vol. 1, p. 172)