There are two errors that are really easy to make when you're using the anchor tag.
First, is to forget one of the quotation marks around the URL value. If one of the quotation marks is missing, many browsers ignore the entire area with the anchor tag and it seems that part of your file has just disappeared. Make sure the URL starts with a quote and ends with a quote.
Second, is to forget to use the closing anchor tag, </a>. If you leave off the tag, your linkand your link colorgoes on until the end of the file. If half your file suddenly changes color, check to be sure you have a closing anchor tag.