Internal hard drive!

@KATXX95  Do you mean you installed a second internal hard drive, as in, you or someone else opened up the laptop and put one inside; or do you have an external drive?  The issue is the same either way, but the best approach to fixing it is not.  The issue itself is that Sims 4 will still look for its user folder in Documents > Electronic Arts, and if the folder isn't there, it will create a new one with no content.  So you'll either need to move your saves, mods, etc. back to that location, or you'll need to tell the game where to look.

If your secondary drive is internal, you can redirect the entire Documents folder to D, which is simple to do and supported by Windows:

https://www.tenforums.com/tutorials/74952-move-location-documents-folder-windows-10-a.html

However, if your secondary drive is external, as in, you could unplug it and start up the laptop without it, you should absolutely not move the Documents directory, because Windows won't react well if it can't find Documents.  Instead, you can use a symbolic link to point the game to your secondary drive:

https://crinrict.com/blog/2020/02/moving-windows-documents-folder-to-external-drive-via-symbolic-link.html

You can also do this with an internal drive if you want, but since it's more complicated, it's usually easier to move Documents instead.

Either way, make a backup of your essential Sims files before you start, just in case you need it later.