The record of Hezekiah's illness, recovery and visit from the envoys of Babylon, is recorded three times in the Bible: here is 2 Chron 31:24-31; 2 Kings 20:1–19; and Isaiah 38:1 - 39:8.
The record we have in 2 Chron 31:24-31 is only a summary of those events. That is 2 Kings and Isa provide much more detail. However, none are complete and each of the three records provides some detail that the others do not. Let me summarize the sequence of events as recorded in all, three accounts:
- Hezekiah become mortally ill
- The prophet Isaiah predicts Hezekiah's immanent death
- Hezekiah prays for recovery
- Isaiah told to tell Hezekiah that he will recover and that as a sign of this, the shadow on the sun-dial will reverse 10 steps
- The sun's shadow does indeed reverse 10 steps and Hezekiah recovers
- Hezekiah composes a hymn of praise to YHWH for his recovery and miraculous sign
- Envoys are dispatched from Babylon because they had heard of Hezekiah's miraculous recovery and seen the reversing of sun-dials - of special interest to the astronomers there; Isa 39:1, 2 Kings 20:12; 2 Chron 32:31
- Instead of explaining the greatness of YHWH, how he had recovered and the associated sign, Hezekiah proudly showed off his riches; 2 Chron 32:31.
- Because he did not give glory to God, Hezekiah is given a dire prophecy of the demise of the royal dynasty; 2 kings 20:14-19.
Thus, the miraculous "sign" was the sun reversing on the great sun-dial of Ahaz (Isa 38:7) which signaled Hezekiah's miraculous recovery.
In commenting on 2 Chron 32:31 Ellicott reaches the same conclusion:
Who sent unto him to enquire of the wonder (Hebrew, the sign, as in
2Chronicles 32:24).—This is not mentioned in the parallel passage of
Kings and Isaiah. But such an inquiry is quite in harmony with what we
know of the Babylonians from their own monuments. Babylon was the home
of the arts of divination and augury, from observation of all kinds of
signs and portents in every department of nature. Moreover, the sign
given to Hezekiah would have a special interest for the astrologers
and astronomers of the Babylonian temple-towers.