If the boundary is sharp and constant, logic is likely but not always.
If the boundary is fuzzy and/or ragged, blockage in the optical path is more likely - a label or Post-It(tm) note that escaped, or even a wad of dust or other debris.
(From: Tony Duell (email@example.com).)
Does the printer have an 'Engine Test' switch (a button on the DC controller PCB on the LaserJet 2, and one on the side of the LaserJet 1)?
This will cause it to print a test page (of vertical lines normally) even if the formatter board (the thing that translates the incoming data into a bit stream to send to the laser) is defective.
If you can find such a button (it's hidden on the side of the lower cover on the Laserjet 2), press it and see what happens. If you only get half a page, then the fault is almost certainly optical (Check for defective mirrors, etc as mentioned earlier in this thread). If you get an entire page of lines, then the fault is in the formatter board. I have no info at all on any formatters other than the Apple Laserwriter 2NT one, alas
(From: Ralph Wade Phillips (firstname.lastname@example.org).)
On one side of the bottom, there's a cover that can be removed after removing one screw. After that, the button is visible through a hole.
Only push this AFTER it warms up all the way - it's ignored until the engine is ready!