A full web browser cache can interfere with Gmail's features and performance. It might make the service slow or prevent you from downloading and viewing attachments.
Please read Clearing your cache for instructions on clearing your cache in any of the popular web browsers. If your browser isn't included, please refer to your browser's help documentation.
In some cases, it may be necessary to clear your browser's cache more than once and/or restart your computer to completely fix the problem.
If you're an Internet Explorer user and are still having problems after restarting your computer, it may be necessary to restore the 'Temporary Internet files' folder on your computer. Instructions are located under the 'Workaround' section of http://support.microsoft.com/kb/301057/EN-US/