X2.... my new L3301 had similar problem the day it was first delivered by the dealer as it was about 6 months on his lot before I got it. I did exactly what you mentioned and in minutes was fine and now years later still good as new.HP, I'd suggest giving the slide track a good dose of PB Blaster or a similar product. Try, as you did, continuing to move the slide. There may just be some rust or small twig, or small pebble laying in there causing all your grief. Probably something simple. Be patient and keep messing with it to convince it to slide.
You've got 17 years on non-movement to overcome! Welcome to the forum.
but since the OP has already beat on it with a mallet something may be bent/broken/jammed over and above.