Desktop, Downloads, Documents...gone?

Try booting to Safe Mode. That boot up takes minutes and clears caches and other things that may resolve your problem.

How to use safe mode on your Mac - Apple Support

Does it work in Safe Mode?

A lot of times just boot to Safe Mode followed by normal boot resolves problems.

The files might still be there Add your Desktop and Documents files to iCloud Drive

Finder >> Preferences >> Sidebar and add the missing elements

If you mean they are missing from the Sidebar as your screenshot shows, click the arrow on the right side of the Favorites header.

There are many other ways to get to those folders.

You can very simply get to them using the Go menu.

So what happened on January 1st? Did you make some kind of change to the computer?

Can you confirm that your downloads are also missing. That is an important detail.

Can you confirm that the files are actually gone? Or are only the sidebar icons missing? Can you find the files by navigating to them using the Finder alone? Are the files listed in Spotlight?

If you were using iCloud Drive Desktop and Documents, then some change to iCloud could make those files go way. You can simply turn iCloud Drive Desktop and Documents back on. But BE CAREFUL with this. DO NOT try clicking on anything in iCloud Drive without knowing PRECISELY what happened or what is going to happen. This could cause irretrievable loss of data and put your computer out of use for days.

What is more important is the Downloads folder. If all of these files are also missing, then iCloud is probably not involved. If Downloads are missing, then it sounds like your user account is corrupt. But here again, you are at very high risk of irretrievable loss of data. You want to proceed very cautiously right now. Do you have a backup? If you do, I wouldn't recommend making a new one right now. You might need to restore instead.

I recommend more details about exactly what might have happened and exactly what data is now missing.