Does my Bootcamp partition, disappeared by Lion install, still exist?
Solution 1:
This doesn't look good for your boot camp partition. Take a look at the drive layout in Disk Utility.app (in /Applications/Utilities, accessible from Launchpad), which visualizes the entire drive in slices. If you don't see the 20GB slice there, then it has been overwritten.
Did you obtain Lion from the App Store?