I used the Disk Cleanup in order to free up some more space in my external
hard drive. The content was about 250 GB, but after using the Disk Cleanup I
had less than 2 GB left.. Only some of the folders are left, and they are all
empty.

This happened because I chose to remove "Catalog files for the Content
Indexer".
According to Microsoft this should not happened:
"Catalog Files for the Content Indexer - The Indexing service speeds up and
improves file searches by maintaining an index of the files that are on the
disk. These Catalog files remain from a previous indexing operation and can
be deleted"

Anyone who could help me with this? How can I get access to my files again?

Thank you in advance. Hope that someone can help me!