A 747 doesn’t cruise at Mach 0.76, nor do many of the commercial aircraft. The 747 tends to cruise at Mach 0.84-0.86. You probably stalled because you were too slow for the aircraft. If you make a flight plan via fpltoif.com, it should give you the cruise speed for the aircraft you are flying, or the list that I’ve linked below may help you as well.
FL380, or 38000 ft, is too high for your weight. It is recommend that you step climb, to prevent this from occurring. In other words, you were too high to support your weight, and therefore the combination of a slow cruise speed and a high altitude probably caused you to stall.
These tutorials below should be extremely useful for your next flight: