message on my i phone is "i phone unavailable". if i erase i phone will i lose my photos?

If you had iCloud Photos turned on you can turn it on again and the photos will sync with the phone. If not sure if it was on, log into www.icloud.com -- if you can see the photos there, that is from iCloud Photos. Also see: See instructions in Set up and use iCloud Photos - Apple Support

If you didn't have iCloud Photos turned and you restore from a backup, the photos will restore. See: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support