StreamReader Exception The Given Path Is Not Supported
===

A while ago I wrote some code that read in a CSV file. This was working fine and I haven't touched it since. I have gone back to test it again and all of a sudden an exception is thrown on this line:

    using (StreamReader readFile = new StreamReader(sFilename))

sFilename is a valid file with the full path to it. It is the SAME file I was using for testing before and is a valid CSV text file.

Why could this have suddenly stopped working?