Can't log into my Webside

Hi there,

for some days now I can’t log into my wordpress-based website. If I try to access it via my “intern”-button after typing in my password an empty page appears. If I try to access my page via wp-login.php it says that “Fehler: Cookies sind wegen einer unerwarteten Ausgabe gesperrt. Hilfe findest du in dieser Dokumentation oder wende dich an das Support Forum.”. So in English it says that “Cookies are blocked because of an unexpected ‘issuing'” – I’m not sure what it means in english.
Well, I really need help with that. I enabled all cookies and did what the support-site suggests but it doesn’t help. Maybe my site is hacked. So is it possible to reinstall wordpress or let’s says to overwrite it somehow? How does it work with the domain then? Should I upgrade to wordpress 4.1?
Help please!

Thank you!!!