Matthew Adams calls it "The Woe" and has six great tips to help you work around it.  Here's his great description:

You have been happily working in the designer, laying out controls, binding in bits of logic, switching between the code view and the designer view, building, debugging. Then, all of a sudden, half your controls disappear, and/or some move to the top left, and/or all your embedded resources (images in particular) vanish without trace... Control-Z doesn't seem to work quite right... The black gloom of despair fogs the monitor, and you're forced to go and get a really strong cup of coffee. Before you start again.

Some folks see this all the time, others not often at all. Word is, this will be a thing of the past once ASP.NET 2.0 ships, but most of us will be dealing with ASP.NET 1.x for at least the next two years, if not longer.

Here is a summary of his six tips and you can get the complete commentary in his post "What has the designer done now?" 

Tip 1 - If you have a saved, corrupted file and are trying to recover from the woe, clean up the orphaned fields so that you can reuse the names.
Tip 2 - Always do a complete solution build before attempting to open any designers.
Tip 3 - Always close all of the designer windows before closing the solution.
Tip 4 - Close all designer windows before doing a build.
Tip 5 - In Case of Woe, Don't Panic. Don't press save. Don't press undo. Just close the designer window (and any applicable code windows) and say "no - I don't want to save the changes".
Tip 6 - Remove the references to the assemblies that contain the controls that vanished, and add them back again.