My dock disappeared in Lion and won't come back unless I turn off autohide. How do I get it back?
Try setting the dock (with auto-hide) to the left or right side of the screen.  If either of those work, it would seem that you may have a screen resolution setting that's keeping the dock off-screen.  

Perhaps try changing your resolution, or bouncing it from one to another.

If none of that helps, you can try deleting the dock preferences (returning it to its default state) and restarting the dock.app


Navigate to (or just spotlight to): ~/Library/Preferences
Delete (or just move them to your desktop) com.apple.dock.plist and com.apple.dock.db 
Open up the terminal and enter the command: killall Dock (if you don't like terminal and killing, you could log out and in again as well)


This will force the dock to make new preference files.