It is probably due to the effects of the local anesthesia that the dentist injected you with before starting the procedure. If you do not remember undergoing that treatment before your crown was fixed however, you should call the dentist and ask. otherwise, the effects of the anesthesia will wear off and your tongue will return to its normal functioning.
Your tongue is numb as a result of the anaesthetic the dentist used when fixing the crown. This happens because the results of the anaesthetic spread from the gums to the tongue. If this wears of within a few hours there is no need to contact your dentist.
Most likely to numb the area where the crown was put in the dentist gave you Novocaine. If given a little to much it can spread to the tongue or surrounding areas in the mouth. It's nothing to worry about, if it wears off in a few hours.
Your tongue might have touched the anesthesia they used to put the crown in your mouth. If the numbness doesn't wear off after a couple of hours, go ahead and call the dentist. Another possibility is that you bite your tongue while you were under the anesthesia. You may just have to wait to let it heal itself.