They considered not the miracle of the loaves - They did not remember or call to mind the "power" which Jesus had shown in feeding the five thousand by a miracle, and that, having done that, he had power also to save them from the storm.
Their heart was hardened - Their "mind" was dull to perceive it. This does not mean that they were "opposed" to Jesus, or that they had what we denominate "hardness of heart," but simply that they were slow to perceive his power. They did not quickly learn, as they ought to have done, that he had all power, and could therefore allay the storm. The word "heart" is frequently used in this sense. See Ephesians 1:18, in Greek; Romans 1:21; Romans 2:15; 2-Corinthians 4:6.

Their heart was hardened - See this explained Matthew 14:33 (note).

For they (z) considered not [the miracle] of the loaves: for their heart was hardened.
(z) Either they did not perceive, or had not well considered that miracle of the five loaves, to the point that the virtue of Christ was just as strange to them as if they had not been present at that miracle which was done just a little before.

For they considered not the miracle of the loaves,.... Which they had seen but the day before; they did not attend to it, nor learn from it, as they might, the wonderful glory of Christ, and the greatness of his power; which was as much an act of omnipotence, as either his walking upon the water, or causing the wind to cease, or more so.
For their heart was hardened; or "blinded"; not by sin, or against Christ, much less in a judicial way: but there was a great deal of dulness and stupidity, and want of attention in them. The glory of Christ, which he manifested, and showed forth in his miracles, was not so clearly and fully discerned, attended to, and acknowledged by them, at it might reasonably be thought it would; for notwithstanding these miracles, which they daily saw, they stood in need of divine illuminations, that the darkness of their minds being removed, they might behold the glory of Christ, as the glory of the only begotten of the Father.

For they considered not the miracle of the loaves; for their heart was hardened--What a singular statement! The meaning seems to be that if they had but "considered [reflected upon] the miracle of the loaves," wrought but a few hours before, they would have wondered at nothing which He might do within the whole circle of power and grace.
Incidents on Landing (Mark 6:53-56).
The details here are given with a rich vividness quite peculiar to this charming Gospel.

Their heart was hardened - And yet they were not reprobates. It means only, they were slow and dull of apprehension.