New iPhone SE, activated less than a week. At least three times now it has, when I wasn't looking, displayed "no sim card" in the upper left, and at least once given a banner warning to the same effect.
Restarting the phone seemed to solve the issue in each case, but I notice a few other threads relating to this model on Virgin Mobile. In other users' experience, could this be a persistent problem?
First, try a forced restart by pressing the Home and Power buttons until the Apple logo appears.
Next try the steps here and, if necessary, contact Apple Support.
If you get to Step 6 remember that a different carrier's sim can only be tried if the phone is unlocked. You cannot simply try a different Sprint sim by inserting it and would need assistance from Virgin Mobile if you need a new sim.
Following my first post, the problem described did not occur again... until this week, and now it seems to be happening with a great deal of frequency.
Once or twice, restarting the phone worked. Three or four times, it did not.
When restarting didn't work, removing and reinserting the SIM card once seemed to do the trick, but only temporarily. Within an hour or two, the phone displayed "No SIM card." Again, I removed and put the card back in, and it's currently connected, but I now have less faith that the problem won't happen again.