So, can you tell us a bit more?
- Are you using iCloud Photos to synchronize your devices? If so
- …Do you have “Optimize Storage” checked in Settings>[name]>iCloud>Photos>
- …Do you see all your iPhone pictures at iCloud.com?
- …Have you checked to see if the Apple Account is really really the same on the iPhone and at iCloud.com?
- …What is your storage in iCloud? At Settings>[name]>iCloud>Storage, mine says 166.6 GB of 200 GB used. What does yours say?
- What is your storage on this iPhone? At Settings>General>iPhone Storage, my phone says 50.5 GB of 128 GB used. What does yours say?
- At Settings>Battery, what do Battery Health, Maximum Capacity, and Cycle Count say?
In the mean time, try a forced restart:
Force restart iPhone - Apple Support
This is different from a simple Power Off. For me, the logo appears 5 or 10 seconds past the "slide to power off" message. Wait for the logo; don't do the slide thing.