something like that happened to my friend but hers like froze when it was taking a nap. She tried to restart it at the back with that little button but it didn't work either. But i think you should try the restart button.
Mine did that too. See if it's on Am or Pm. Cause My Tamagotchi use to do that and I din't know what to do. It was sleeping in the day and would wake up at night. The I changed P.m to A.m And now my tamagotchi wakes up at the correct time. Hope I could help!