some of my photos have disappeared.

O.k. I can now see that you are using iCloud Photo Library and "Optimize Storage".

Are you using iCloud Photo Library on another device as well and perhaps delete photos on other devices? That will delete them from iCloud Photo Library and all devices using iCloud Photo Library.

The "Optimize Storage" option has to be used with care. It has the following consequences. when your iPhone needs free storage, it will remove photos you have not used recently from the iPhone and keep it only in iCloud. You may just be seeing a thumbnail, and this thumbnail will be blank, if you do not have much free storage, or if the iPhone cannot connect to Wi-Fi.

On your system version iOS 11.2.1 , even photos you just have taken may become optimized. This strange behaviour seems to be fixed in the most recent update. You may want to update to the newest version of iOS.

How much storage is available on your iPhone X? If you have many more photos in iCloud Photo Library than will fit onto your iPhone without optimization, I would reduce the amount of photos in iCloud by saving them to a computer and keep only the favorites in your iCloud Photo Library. Then you can disable "Optimize Storage", if all remaining photos will fit onto the iPhone.

Are you running out of storage on your iPhone?

Are you using iCloud Photo Library on your iPhone and on another device?

Check the Settings > Your Name > iCloud > Photos, if iCloud Photo Library has become enabled or Optimize Storage. Has it been turned on?

So how can I get my dissappeared photos?

Before I can recommend something, I need to know, how much free storage you have on your iPhone. Is the iPhone full? How many photos are in iCloud? Would they all fit onto your iPhone? Or are more photos in iCloud than you can save on the iPhone? Is your iCloud storage full?

You can see the amount of storage and the capacity in Settings > General > About.

And Settings > General >Storage will show you how much is currently used for photos on the iPhone and in iCloud.

To check, if your missing photos are still in iCloud: Do you have a computer or can borrow one? Then you can view your Photos Library in a web browser.

Go to the web page www.icloud.com . Sign in with your iCloud AppleID and open the Photos.app there. It will show you all photos you are having currently in iCloud. Are your photos still there? Then you can download them from there to the computer to have a backup copy.