I have a feeling that your cat is not eating because of the pain where her tooth was extracted. She may not want to eat until the pain is relieved, but she may be able to eat soft, canned food.
Lethargy is a normal side effect of diazepam. However, IV diazepam has a duration of about one hour in humans, and I can't imagine it is too much longer in cats. It shouldn't really still be in effect a day later. Your cat may be lethargic due to the pain. If things don't change, call your vet.
