Unable to login into Admin Panel after upgrade

Hi,

I upgraded my website from wordpress 2.8.6 to wordpress 4.1 through upgrade feature on wordpress website. After the upgrade I can view my website but I can’t login into admin panel. Admin Panel appears but when I give my username and password and press the enter the button, the page refreshes and again I see admin panel. I have tried disabling all the plugins and themes but have not been successful.