Check your paths, Best practices is to always use a relative paths. Ex
If your stylesheet is called style.css the link should be:
<link rel="stylesheet" type="text/css" href="style.css" />
If you have the css file in a subdirectoy (and the subdirectory is called styledirectory) the link becomes:
<link rel="stylesheet" type="text/css" href="styledirectory/style.css" />
If the css file is in the parent directory of the html file the links becomes:
<link rel="stylesheet" type="text/css" href="../style.css" />
.. goes up 1 directory, if you need two you could do: ../../
Sample Folder structure

Assuming structure is like that then,
<link rel="stylesheet" type="text/css" href="styles/BGsampleSS.css" />
Name files without spaces too.