You're not talking to Apple in this forum, we're users just like you.
Assuming you made a backup to itunes or to icloud, then you must do a restore of that backup (assuming it still has the contacts that you seem to have lost) to the device.
Go to Settings>General>Reset and tap Erase All Content and Settings. This will erase your device. Then you will go through the setup screens again as you did when your device was new, and when given the option, select Restore from iCloud Backup.
This link gives another overview of backup and restore:
HOWEVER... you may run into a snag, namely when the newly restored data is downloaded, an automatic icloud sync may occur that will wipe out any Apple data (contacts, calendars, etc) that was restored. If you have such an issue, read the following that describes restoring contacts. The info will also pertain to other Apple data.
It's always good idea to back up. It's even better if you keep written record of your contacts, in case technology lets you down. The good old way, works. I have personally NEVER lost any iPhone contacts; but I do know you can get iPhone contacts back, with tutorials like these: How to get back your iPhone contacts. You do need to check all specs and compatibility etc.,
Or you can check some of the other suggestions made in the similar thread.