If you mean the device is not recognized in iTunes, the first thing to try is quit iTunes, restart the Mac and restart the iPad and then try again.
If you don't see the iPad in iTunes enable the sidebar by going to View>Show Sidebar from the iTunes menu. Once the sidebar is enabled, you should see the iPad under Devices in the left sidebar.
If that's not the problem and iTunes is not recognized at all, look here for help.
If iTunes is telling you that you that your iPad will be erased when you sync, you have to either transfer the iTunes library from the Windows machine to the Mac, or if the old computer is gone or not functioning, then you have to make the iTunes library your syncing library, by following the steps in this user tip.
Sync Your iOS Device with a New Computer Without Losing Data
Syncing to a "New" Computer or replacing a "crashed" Hard Drive
iPad not appearing in iTunes
iOS: Device not recognized in iTunes for Mac OS X
iOS: Device not recognized in iTunes for Windows
iTunes for Windows: Device Sync Tests
IOS: Syncing with iTunes
Apple - Support - iPad - Syncing