(1) Are you using the secure https:// login?
http://en.support.wordpress.com/https/ If not then try using it.
(2) Exactly which browser and version of it are you using. If you aren’t sure click here to find out > http://whatsmyuseragent.com/
Note 1: If you are using an IE9 browser in compatibility mode that makes it render as IE7 and IE7 is not supported. To disable see > http://www.sevenforums.com/tutorials/1196-internet-explorer-compatibility-view-turn-off.html
a. Try clearing your browser cache and cookies > http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=32050
b. Try using the browser with all extensions or add-ons disabled
c. If applicable try updating the browser to the latest stable version http://browsehappy.com/
d. Make sure third party cookies are enabled on the browser http://support.google.com/accounts/bin/answer.py?hl=en&answer=61416
f. Then confirm Flash is updated http://www.adobe.com/software/flash/about/ and if required installing the latest version of Flash http://www.adobe.com/products/flashplayer/
g. Try resetting your router and modem and rebooting your computer and then logging in using the secure https:// log-in
h. Try using another browser. You can download one here http://browsehappy.com