Lost half my contacts, how can I restore??

How did you lose them? Do you sync your contacts with anything (iCloud, Google Contacts, a contacts app on your computer via USB cable, etc?) Do your contacts come from more than one source? Do you have an MS Exchange email account on your phone that syncs Exchange Server contacts?

The most common cause of missing contacts is having multiple contact groups where not all are selected. To check for this, open the Contacts app, tap on Groups in the upper left, and verify that all groups are selected.