3 Solutions Ranked from Good to Better to Best
GOOD: Pool Putty or Epoxy Patch the Crack. This is the cheapest way but there is a high likelihood (almost 100%) of the crack coming back because the underlying issue (shifting shell) is not being addressed.
BETTER: Refinish the Entire Pool Surface. Very expensive (6-9K) but must be done by highly experienced personnel and money spent may be wasted (roughly 40% chance) because the underlying issue (shifting shell) has not been addressed.
BEST: Install 30 Year Pool Membrane Over Entire Shell. Comparable in expense to resurfacing but continued shell shifting will not effect the flexible membrane. Definitely the best choice if you can afford it.