It saunds like you have taken sensible steps to fix it. Have you left the shortcut for the lasso tool checked in the Edit space? If so, does toggling through the 'l' shortcut locate it?
Another thought is custom workspaces. If you have saved a custom toolbar with a custom workspace, that might be hidding the lasso tool. Try selecting Essentials, and then resetting essentials.
I guess a first step might also have been to select the lasso tool from the edit space, and reset it from the left end of the Options bar. A last resort is to reset Preferences of course, but I hate doing that unless absolutely necessary.
Good luck.