New harddrive, cannot sync my iPad
As I referred to in the comment:


Right/(Control)-Click on iOS device in sidebar of iTunes.
Select Transfer Purchases


This copies over everything you purchased using the iTMS, App store, and iBooks.

For all content you put on the device that you purchased/received elsewhere, the process is a bit more complicated. Here are some suggestions:


Right/(Control)-Click on iOS device in sidebar of iTunes and select Backup. These at least gives you a place you can quickly fall back on.
If the computer in-question is a Mac, I can't recommend Podworks enough. 
If Windows... well I haven't tried any, but found this thread on SuperUser.


Hope this is enough to get you going. Best of luck!