You should provide more details about your situation.
Have you made any changes to the website lately? Have your backlinks changed? Has your traffic gone down or just your rankings? How long has your website been experiencing this? Has your competition increased or improved? Why do you think this might be a Google penalty? Have you bought any links? Have you checked for duplicate content issues? What have you done to diagnose and fix the issue?
Asking why a website has lost rankings is too general of a question. Your question is too hard to answer without you providing more information.