Welcome to the Q&A Forum

Hi, Our website is well established and has been growing every month until around Jan. 6th 2013. All of sudden our search rankings on Google dropped dramatically (nearly 50%). We getting tens of thousands fewer visitors through search results than normal. At the same time as the drop (almost to the day) we changed our forum software from Vbulletin3 to Vbulletin4. The forums are on a different subdomain, and our forum developer said there is no way the change would affect the main site. Is that true? So  far, we have not been able to figure out what caused the sudden change. Maybe a Google update?