Dear Rabbi,
Q. What is the correct course of action if one woke up very late on Shabbos morning, but it was not because of an oness, but was because, for example, you stayed up too late? Say one wakes up much past the latest time for Tefillah, but it is still before Chatzos? Should you daven shacharis? A condensed shacharis? Is there an inyan to finish Amida before Chatzos at least?
Thank you.

A. Mishna Berura (89: 6) rules that one should daven shacharis after four hours until chatzos. However, he recommends that one should do so as a tefilas nedovo (voluntary prayer), since there are Poskim who maintain that if the reason for the lateness was not accidental, one should not daven anymore. The above does not apply on Shabbos, since tefilas nedovo is not recited then.
As far as reciting the brochos of shema after four hours, Mishna Berura (58: 26) rules that even if it was due to an oness or an unavoidable circumstance it is prohibited as the blessing may be in vain.
However, in Biur Halocho (ibid.) he quotes Mishkenos Yaakov that is lenient in case of oness.
So after the fourth hour if one has time until midday after the fourth hour, he may say all the pesuke dezimara with the corresponding complete brochos of baruch sheamar and yishtabach. However, if he should or not recite the name of Hashem on the brochos of shema, would depend on the above opinions. Horav Shlomo Miller Shlit’a indicated that the minhag seems to be to say with the name. One should say the complete shema.
If pressed with time, he should avoid the blessings of the shema and he may shorten the pesukei dezimra.
One should also aim to end the amida completely before chatzos. If that is not possible he may start even if he wont be able to finish before midday. (Yavia Omer O.H. 7: 34, Vayaan Dovid 1; 19, et. al.)
Horav Shlomo Miller’s Shlit’a opinion is similar. In regards to considering staying late a mezid or intentional. The Rov maintains that it is a peshia or a negligent case close to being mezid, but still permitted to daven as above.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a