A few days into a new project, suddenly flex builder 3
crashed and I can no longer graphically edit my layout. Editing the
source works, and the changes are reflected in the design window,
however I cannot make changes while in the design window.
You can try to delete (not contents), then reimport your
project, or simply reinstall Flex. Takes all of 5 minutes. If that
still doesn't work, make sure you don't have some component
covering the others possibly.