We can't help you.
It seems your file association to PDF files has been altered. Since this is an Adobe Reader forum, your question would be best answered in a Microsoft forum, if in fact you are using a version of Microsoft Windows as the operating system on your system. It's a relatively simple fix but since I do not know what version of Windows you may have, the best suggestion I can make is to Google "modify windows file associations" without the quotes. Your answer will be in one of the first few links. Check support.microsoft.com for further issues of this nature.
Best of success!