Shoutbox

Things that usually help in this situation;
- Clear your browsers cookies -
Firefox: Tools > Options > Privacy > Cookies > Clear, also try clearing the cache using the same method, just select cache at the privacy screen and clear it.

IE: Start > Control Panel (Make sure classic view is selected) > Internet Options > Delete Cookies > OK. Also click "Delete Files" next to it and click ok to the following message.

Opera: Go to Files on the tool bar. Click on Delete Private Data. Then select what cookies you want to delete. Usually, a cookie has a name that is the same as or similar to that of the web site that provided them.

If doing that in your browser doesn't work another option you could try is to try using various browsers, and see if it works in other ones.

And of course an old favourite - reboot your machine, if there is some dodgy setting stopping you accessing Gmail properly this may correct it.