Contacts disappeared from iPhone

I'm helping a friend with an iPhone issue. All her contacts disappeared from the iPhone, but were still on the computer. After syncing with iTunes, contacts still not on iPhone. Tried on the Info tab, under Advanced, to check "replace contacts on iPhone" but it was grayed out. Did a restore, update, and then sync, everything else is OK but still no contacts. Finally able to check the "replace contacts" box and synced again, contacts finally showed up on the iPhone. Any idea what happened?