Yikes! Yes, it does matter, very much, that there is already an .htaccess file already there!
As you can only have one .htaccess file in any directory, if you do that, the "new" one will "overwrite" the other one. In this case, that will "disable" your .htaccess based password protection.
Not to worry, there are (at least) two ways you can deal with this problem:
1) You can combine the contents of the two .htaccess files into a single .htaccess file
2) Since your first .htaccess file was designed to provide password protection for a directory, you can leave it in the directory you want protected (assuming that it is a subdirectory of your main site - as in yourdomain.tld/private) and place the .htaccess file that denies access to given IP addresses in the directory above that one (as in yourdomain.tld). That way, the .htaccess file in the subdirectory will provide password protection for the directory it is in, while the .htaccess file in the parent directory will prevent access to the whole site from those IP addresses.