If you need an answer to your problem how to make iPhone unavailable fix, first of all, consider whether your screen allows some time limit or erasing options. Do not enter passcodes randomly since each wrong attempt will extend your waiting period. Resetting your iPhone may be a quick solution if you had made backups in recent times. But when resetting your phone, you can lose all your locally stored data that was not saved in your backup copy. That is why it is necessary to consider what you have on your iCloud Photos, iCloud Backup, etc., before erasing your phone. In case your problem is restoring lost data which is impossible to get back from any backup, you should think about the use of some special software. The information about iPhone data recovery solutions you can find on Datarecovee.
Do regular backups to prevent data loss.