Everytime I press D + LMB I see the pencil as cursor and the indication that I'm about to draw something, but every time I draw a line, the line disappears.
Check to make sure you don't have the [Grease Pencil layers] and [Grease Pencil Colors] set to invisible (eye icon). I spent an hour trying to figure out why, but forgot that I turned them off.
For the new players: N to open the right hand menu. Uncollapse the ▼ ☑Grease Pencil and ▼Grease Pencil Colors Look to make sure the eyes are visible for both layers. Also try creating a new Grease Pencil layer and seeing if that fixes any issues.
Double check the opacity of the active layer. If it's set to zero, the created stroke won't be shown after drawing, by it will be visible when drawing
I met the same problem today.Maybe you pressed H accidentally. Go to the edit Mode and then try press Alt+H see if it works.
Check to see that you didn't accidentally go to a new frame in the dope sheet.