I'm using ACAD 2009 and my line command is somehow not working. Maybe I have turn off some toggle on the keyboard.  I'll enter l command, pick a first point then move to a second point, enter, and the line will not remain. Thanks for any help

Please note that L is not a command, just a shortcut (alias) so it depends how you have it mapped. The the standard _LINE command.

Is your current layer turned on?

Are you sure you're not drawing on a layer that is turned off?