It's easy to accidentally delete a contact--you might press the wrong button or a small child might do that for you. You can recover a deleted number only if you have backed up your phone with a desktop software manager or through your carrier's backup application.
Smartphone Desktop Manager
Double-click the "Desktop Manager" icon on your computer to open the program.
Connect your phone to your computer with the data cable.
Click "Backup/Restore," and select "Restore Contacts."
Carrier's Backup Service
Select "Menu" and scroll through the applications to find your carrier's backup application. Click it to open it.
Enter your password when prompted and click "OK." Select "Yes" when the program asks if you want to restore your contacts.
Wait for the program to indicate that your contact restoration is complete.
You can download your carrier's backup application to your phone from the carrier's website or via the "Get New Applications" option of your phone. Once you download the application, it will ask you if you want to backup your contacts. Select "Yes" and the carrier will back up your numbers to a server.