I am very sorry that this happened to you! Losing work that you have spent time working on can be very frustrating. I assure you that I will try my best to find the cause of this issue.
In order to do this I would need you to provide answers to the following questions:
What web browser (e.g., Internet Explorer, Safari, Chrome, etc.) and operating system are you using?
Do you connect to the Internet through a work network or router?
Who is your Internet Service Provider (ISP)?
What changes were you making within the campaign when this occurred?
Once I have these answer I can definitely suggest some steps to take next. Also please take a look at the troubleshooting steps listed here if the issue is happening constantly when you are using Constant Contact.