To wake a sleeping Tamagotchi, you need to manually change its sleep clock. This is done by first ascertaining its sleep schedule and then pressing a few buttons.
Find your Tamagotchi's sleep schedule
Tamagotchis have preset sleep schedules. Depending on the type of Tamagotchi, it will go to sleep at either 8:00, 9:00 or 10:00 p.m. and wake up at either 8:00, 9:00 or 10:00 a.m. Refer to your manual if you need help figuring out what time of the morning your type of Tamagotchi will wake up.
Find your Tamagotchi's clock
The Tamagotchi device has an internal clock that tells the Tamagotchi what time of day or night it currently is. Locate the clock application by pressing the "B" button. The current time should appear on your screen.
Change your Tamagotchi's clock
While the time is still visible on the Tamagotchi's screen, hold the "A" and "C" buttons down simultaneously. You should now be able to change the time by pressing the "A" button. When the clock shows the time your Tamagotchi wakes up, press the "B" button to confirm and then the "C" button to resume the clock. Your Tamagotchi should now be awake and ready to play.