It is really a good habit to back up your iPhone data periodically on your computer. That will save you when your device need erase data for repairs, or erased the data accidently, or file corruption. What you should do it just to connect your device to the computer and restore your data from the backup. It is recommended you to make the same backup on all the computer you are using as any sync operation will erase the data on your iPhone and replaced with the backup. Or you’d better use only one computer. Below I will introduce you how to accomplish restore.