It is highly unlikely that your iPad will have been “hacked”.
While all computers have unpatched vulnerabilities, in the highly unlikely event of a successful exploit occurring, the exploited vulnerability is very unlikely to be successful in altering any of your data.
In view of Apple’s advanced security architecture, it is far more likely that you have experienced data-corruption - or have unintentionally modified your address book.
Ensure that your iPad has been updated to its most recent version of iOS/iPadOS. Here is a list of the highest version of iOS supported by all models of iPad (for simplicity - “generations” are shortened to a number) that are restricted in their ability to be updated:
iPad1 - iOS 5.1.1
iPad2, iPad3 and iPad mini1 - iOS 9.3.5 (WiFi Only models), iOS 9.3.6 (WiFi & Cellular)
iPad4 - iOS 10.3.3 (WiFi Only), iOS 10.3.4 (WiFi & Cellular)
iPad Air1, iPad mini2 and iPad mini3 - iOS 12.5.6
Newer models, not appearing in this list, are all able to be updated to the current version, iPadOS 15.7
Not sure which iPad you have? Identify your iPad model.