How do I get back photos that disappeared?

If you have pictures that were transferred from a computer, pictures that can be viewed in Photos but can't be modified by Photos, then when you turn on iCloud Photos there is a warning about those being at risk. So if your missing pictures were transferred from a computer, you should look on the computer. 

You can see this:

         If you're missing photos or videos in the Photos app - Apple Support

Basically the above says to look in Recently Deleted and in Hidden. Be sure to check your Library thoroughly-- the pictures may not be where you expect, as dates may have changed. You can use the search field with the date or location. And remember, you can sort the Library by date captured or by date added. Also, your Library view now has filters, and you may not have your filter set to "All Items."

If you don't use iCloud Photos, but you have Settings>Apple ID>iCloud Backup turned on, then you may be able to recover data from the last 30 days or so. You should look here:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support 

“Restore” is what you do when you buy a new phone, so this safety, if you have it set, requires you to erase everything on your phone and replace it with the backup.

If you don't keep your own backups to an external drive, then there really isn't any place else to look.

How were you managing your photos? Were you using iCloud? Do you have a backup of your device? If you were using iCloud, did you check the Recently Deleted folder? Photos should not just disappear on their own, but hopefully you have a backup you can rely on? If iCloud, then log into iCloud on a computer and see if the photos are still there.