2 Samuel 6:23

23 Therefore Michal the daughter of Saul had no child unto the day of her death.

2 Samuel 21:8

8 But the king took the two sons of Rizpah the daughter of Aiah, whom she bare unto Saul, Armoni and Mephibosheth; and the five sons of Michal the daughter of Saul, whom she brought up for Adriel the son of Barzillai the Meholathite:

Did Michal, who was at one point David's wife, die childless, or did she have 5 sons?

One resolution simply takes the text at face value - which says Michal "brought up" the five sons of her sister, Merab (1 Samuel 14:49), who was married to a man named Adriel (1 Samuel 18:17-19). After Merab presumably died, Michal raised them, with a man named Phalti (1 Samuel 25:44).

Another resolution is that Merab herself, not Michal, is named in some Masoretic, Septuagint (Greek Old Testament), and Syriac manuscripts in 2 Samuel 21:8 (E.W. Bullinger), which would mean the proper reading should be Merab, and this only represents a mistake in textual criticism.