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Where did you have your contacts saved? If you only had them saved in the local Phone account, they don't get backed up or synced anywhere automatically, which is why I always strongly recommend saving contacts to your Google account (or Samsung account), because those will get synced automatically to the cloud.
Regarding your photos, try using a file manager app and looking directly in the /DCIM/Camera directory to see if the actual photo files are still there. Just because a gallery app no longer shows photos doesn't necessarily mean they're gone, because gallery apps are simply for viewing -- they don't actually store anything. If something glitched where the gallery app no longer knows where the photos are, it may seem like all the photos were lost.
Also, are you sure you didn't' have Google Photos automatically backing up your photos? Go to photos.google.com on your computer browser and see if they're there. The free unlimited backup feature is going away in June, but until then, you can still back everything up at almost the original quality (assuming you don't have a Pixel phone up through the Pixel 3, which can still back up photos at original quality). Another good option is Amazon Photos, which is unlimited original quality as long as you're an Amazon Prime subscriber.
You should still be able to screenshot by pressing Power and Vol Down together for about a half second. Depending on the phone, there may be a couple of other ways to do it, but we'd have to know which phone you have. What happens when you try to do the Power/Vol Down press?
I'm not sure about WhatsApp or Snapchat, because I don't use those apps. But it's worth clearing the app cache and data for those apps and doing a Force Stop, then opening them again, signing back in, and seeing if the same thing persists.
Depending on the phone, it may also be worth wiping the cache partition, which can be helpful after a major system update.
Threatening to move to iPhone doesn't faze us -- you do what you need to do. But it's still worth trying to troubleshoot. (iPhones are excellent devices, but they are not without their glitches as well.)