Since the iPhone is unavailable, the only option is to restore the iPhone. After that you will be able to restore your latest backup to restore your content. If you do not have a backup, your content will be lost, except for content you are syncing with iCloud, such as Contacts, etc., whatever you have turned on to sync in iCloud. This would include photos if you are using iCloud Photos to sync your photos across devices. I certainly would not clear out your iCloud via PC, as that will remove the ability for you to turn iCloud back on to sync your content. Not all content syncs with iCloud, so you would also have to restore your backup, whether iCloud or iTunes to get the balance back. If you forgot your iPhone passcode - Apple Support
Since the iPhone is unavailable, you will not be able to back it up now. Backups have to be made before you experience a problem with the iPhone. If you are using a regular backup schedule, you should be okay.