This can be caused by a couple of things:
-You set the clock wrong
Push the B (middle) button, then when you get to the time screen press A (left) and C (right) at the same time. Then where the little hearts were counting the seconds before it now says "set".
Using the A button you can change what is flashing and using B you confirm that which you set. When the time is set right, press B again and "set" disappears.
-Your tama is taking a nap
But this is not usually for hours, mostly for an hour max
-Your Tama is asleep
Most Tama's fall asleep at around 20.00 or 21.00 and wake up again at 08.00 or 09.00