If you have AdBlock running please try disabling it.
If you are not using the https:// secure log in please try using it.
Please clearly state whether or not you are logging in on the wordprress.com home page or you are logging in directly to your own blog.
These are the troubleshooting steps we take whenever any issues like this arise:
clearing your browser cache and cookies > http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=32050
using the browser with all extensions or add-ons disabled
updating the browser to the latest stable version (not a buggy BETA browser version) http://browsehappy.com/
confirming third party cookies is enabled http://support.google.com/accounts/bin/answer.py?hl=en&answer=61416
confirming 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/
resetting your router and modem and rebooting your computer
then logging in using the secure https:// log-in see http://en.support.wordpress.com/https/
trying another browser
If those do not work for you then we need to know
(1) how you are connecting to the internet and to WordPress.com. The way you connect to the internet (mobile, satellite, DSL, dial-up) and to your blog, and how many proxy server jumps etc. it takes to connect can cause problems. There maybe be problems with your ISP and/or with the proxy servers. Also note that the main dashboard isn’t designed to work with touch devices through the tablet and smartphone browsers.
(2) exactly which browser and version of it are you using. If you don’t know click here to find out > http://whatsmyuseragent.com/
The more information you provide to Staff the more they have to work with. I have flagged this thread for Staff attention.