Probably the biggest reason for a turtle not eating is the temperature being
too cool in its surroundings. Turtles are also known for being
very picky eaters so you may want to try a different type
of food. They become bored with their food very fast which
is why it is suggested to feed them a variety of food.
Like most reptiles turtles are very susceptible to stress.
If you have just changed something in its habitat or moved
it altogether it is not uncommon for the turtle to take time
to adapt to its new surroundings. If your turtle seems to
have a lack of appetite during this time you probably have
nothing to worry about just ensure that it goes away after
a few days.
If all of the above suggestions fail you will have no choice
but to take your turtle to see a veterinarian. The veterinarian
should be able to determine the root cause and offer you some
suggestions to make your turtle feel more comfortable. Nutritional
deficiencies can commonly occur when the turtle does not
eat which makes it important to act immediately.