If you find the Send button is not working / not responding, this is usually a browser cache issue.
Upgrades / maintenance typically take place on a Wednesday at the end of the month. If you are logging in after an upgrade, there is a good chance the files on your local machine are out-of-sync and need clearing / refreshing.
For instructions on how to clear your browser cache as well as other troubleshooting topics, please click here.
If you have had MX open a long time on the Send page, you may have been logged out.