We're really sorry to hear when customers lose data that they've been working on for a long time. Sometimes, especially when you've been working in the editor for a long time, you could experience a loss of connection that causes the editor to crash. Press the save button on the top right of your editor page periodically and verify the "last saved" time to make sure that it has been saved recently to avoid this problem in the future. If you have auto-saved turned on, please still make sure to look up at the "Last Saved" time to make sure that you haven't made significant changes since that time that would be inconvenient to lose. Unfortunately there is no way to revert back to an unsaved version of an email. Again we're really sorry to hear when people lose work because it's a very frustrating experience but at this time we can only give you advice to avoid the problem in the future.
Besides making sure to always press the save button, people often experience issues with our website if they are using old browsers or have a lot of stored information in their browser that is causing sluggish performance. Try clearing your cache and cookies and making sure that you have the latest version of your browser installed to avoid this in the future.