Should you encounter problems with your site, where you do not have a support plan, please do not e-mail or telephone, you should first of all checkout troubleshooting tips on the free support forums here and if there are notifications of similar problems from other customers, you will see these - this of course avoids any unnecessary repeat communications.
If troubleshooting does not resolve your issue, you should post the issue on the relevant forum, giving as much detail as possible - ie. what you were doing/trying to do, what error message you saw, include a screenshot where possible. We will respond via. the topic if it is a problem that appears to be affecting all sites, we will investigate and report back via. the forum.
If you have a support plan in place, by all means check the forums for known issues, and of course the recommended troubleshooting tips before telephoning. You can of course e-mail or use live support/leave a message - see bottom right.
You need to give as much detail as possible (check through all the troubleshooting tips first though!), eg.
The equipment you were using: ie. laptop, desktop, tablet, mobile phone
The browser you were using: Internet Explorer, Firefox, Chrome, Safari
The URL you were trying to access - you can copy and paste this from the address bar
What you were doing/trying to achieve
Any error message that you might've seen (copy and paste)
Include a screenshot as an attachment.
State the last time that this was working for you?
State whether you have updated your Joomla sites or any plugins (if you have, did you restore your site from Backup?)