1) test your water and post it: ammonia, nitrites, nitrates and pH.
2) give the size of the tank, the size of the shark and the types and sizes of all the other fish in the tank
3) tell us what you are feeding it
4) tell us if you saw the shark eating the same thing at the store
5) tell us what foot traffic (people) and sounds (music, cars, tv) are around the tank
Having said all that, it's not uncommon for fish to take a while before they eat. But the above information would help clear up some easier issues.