You can check your logs to go back to a previous point in time.
The following steps assume you deleted the contents and then made commits after.
To find the right commit, first check the history for the deleted file:

$ git log -- <filename>

You can either work with the last commit that still had the file, or the commit that deleted the file. In the first case, just checkout the file from that commit:

$ git checkout <commit hash> -- <filename>

In the second case, checkout the file from one commit before that:

$ git checkout <deletion commit hash>~1 -- <filename>

If the file was never actually committed this of course will not be in the history.

You mentioned:

Then on my other PC, there was git repo containing all the code of FBA_Matting with some changes, So to avoid fatal: refusing to merge unrelated histories

With the folder added this changes the history, so you will need to address the differences between the files and fix the merge conflicts.

Use :
$ git pull origin [repo] --allow-unrelated-histories

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