Denied "access /wp-admin/admin-ajax.php" (htaccess) ...

Hi,
I cannot log into my website anymore, receiving the following message:

"You don't have permission to access /wp-admin/admin-ajax.php on this server. Server unable to read htaccess file, denying access to be safe. Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request."

The last actions I did before this were : 1) running snaphot backups and 2) upgrading my hosting from "shared" to "virtual private server" ...

Also, some weeks back, I had uploaded an .htaccess file with the code to establish a multi-site network (as I could not find one) ... thinking now that this might be the problem though it was actually working fine so far ...

Trying to fix the .htaccess, I can view the files in my CPanel, File Manager (Search) but double-clicking, it would not open ... see attached screen.

Many thanks in advance for your help, Elisabeth