Cannot access my dashboard after upgrade

I upgrade my site: http://www.wendimomen.com called Wendi’s Wanders which I have had for several years, although I have been inactive for the last couple of years. The site is still accessible to view but when I try to back into the dashboard, I just get a white page and no one seems to recognize my user name, including on this site (hence I have re-registered just now). Why has this happened, how can I fix it? Thanks