For quite some time now, we are getting sporadic reports of mysterios login problems, mostly with Internet Explorer but sometimes with other browsers on other OSes.
There are a few known problems, and those include not being able to logged in to multiple sites on the same domain. The fix to this is easy <code>ini_set('session.name', 'sitename')</code> . Also we are getting reports that sometimes the login does not seem to succeed -- just press reload.
Now, if you tried to log in to just one site, and not even reload shows you logged in, you are facing this problem. If you can reproduce it reliable and want to help your fellow Drupalers who face the same problem and know one end of a text editor from the other then please comment on the issue. I will lead you trying to find a solution. First advice: try to reproduce with another browser on the same OS. If it happens with two browsers, then it's more likely an underlying infrastructure issue, which I won't detail here. However, if it does not happen, then reinstall the browser. If it's OS-bound then reinstall the OS. Try to reproduce again. If not succeeded, install back all the crap that your browser had and try logging in from time to time. When it breaks, then post here.