support.apple.com/iphone/restore

That's unfortunate, but the time to ensure you don't lose data is BEFORE something goes wrong. If you see the restore screen, you have to restore your phone. And the link which brought you here contains the directions you must follow to do that.

You have now learned why backing up your phone is very important and how not backing it up can be really bad.

You have no choice. All of the content of your phone is already gone. You must restore to factory settings if you want to use the phone.

Backups must be created before you need them; after is too late.

If you received the restore message after following the steps and selecting update (instead of restore), you may have no choice but to restore the phone and erase all contents. I'd see what others have to say and be sure that you tried the update option (if available )