Unable to find my files, but they take up space

I have a strong suspicion you have another partition mounted on /home. When you booted to a live USB, you didn't mount that partition, so files copied to your home directory were copied to your home directory on the partition mounted on /. When Ubuntu boots and mounts something on /home, the old contents are effectively inaccessible until you unmount /home.

You can bind mount / elsewhere to access the directory masked by other mounts:

mkdir foo
sudo mount --bind / foo
du -sh foo/home

If you open a terminal and cd / then this command

sudo du -aBM -d 1 . | sort -nr | head -20

will show you the 20 largest directories (including subdirs) in your file system - it does take a while to run the first time. You can then change to what looks like a larger than expected directory, and run the command again, looking for again bigger then expected directories, and hopefully you will eventually find your files.

chick@dad:/$ sudo du -aBM -d 1 . | sort -nr | head -20
[sudo] password for chick: 
du: cannot access './proc/30738/task/30738/fd/4': No such file or directory
du: cannot access './proc/30738/task/30738/fdinfo/4': No such file or directory
du: cannot access './proc/30738/fd/3': No such file or directory
du: cannot access './proc/30738/fdinfo/3': No such file or directory
du: cannot access './run/user/1000/gvfs': Permission denied
655023M .
645220M ./home
5481M   ./usr
2049M   ./swapfile
860M    ./lib
668M    ./var
491M    ./opt
201M    ./boot
17M ./sbin
16M ./etc
15M ./bin
11M ./run
1M  ./tmp
1M  ./srv
1M  ./snap
1M  ./root
1M  ./mnt
1M  ./media
1M  ./lost+found
1M  ./lib64

Step 0. Read man locate.
Step 1. Ensure the locate database is up-to-date:

sudo /etc/cron.daily/locate  

This will take several minutes - have patience.

Step 2. Use locate to find one of your files that has a unique-to-you name. E.g.:

locate GeorgeRomero_Birth_Certificate  

(if your name is "George Romero". Mine isn't, YMMV.)

Step 3. Read man mv and consider using mv to move a whole directory structure to where you want it. E.g., if the Step 2 locate showed:

/some/where/Documents/GeorgeRomero_Birth_Certificate  

you could do something like mv --target-directory=$HOME /some/where/Documents.