We bought a pool in 2000 last winter my nephew knocked some holes in it. The pool completely drained and we took the liner out to clean it and patch up the holes.
Can we reinstall the liner?

No, the liner is to old and brittle and has shrunk. You will have to replace the liner

Ok so we cleaned the pool really well and then patched the holes in the pool. To my surprise after reading all of the other answers on here I honestly do not understand why everyone says that it can't be done! YES IT CAN BE DONE IT JUST TAKES PATIENTS AND TIME!
We even went to a local pool dealer and they told us that it can't be done, boy oh boy was he surprised!

You said it was a 2000 pool but did you install a new liner when you set it up in 2008? A liner only a few months old is not stretched or dried out but a 2000 liner sure would be.