I have spoken to several people who have lost all their contacts on their iPhones/ iPads and it seems to all be the same problem. Nobody knows where their contacts are actually stored. If you setup your phone and only ever had one contact source, then you are probably ok (check anyway). But if you have several mail addresses all on one device, its time to check where your contacts are located.
If you are an Exchange user this will be extremely important. Lots of comanies are opting for the iPhone as apposed to blackberry’s etc. The problem here comes in when an employee, decides to add a iCloud, Gmail, Hotmail or any other mail address. Under Settings – Mail, Contacts, Calendar you will be presented with options on where to store your contacts and Claendar events and what mail should be used as default, I bet there are thousands of employees sending mail from Hotmail instead of their work account.
If you click Settings, and go into your Mail, Contacts and Calendar have a look at what your defaults are, you may be suprised.