It sounds like you did some internal damage. What that damage is, is hard to tell.

If you look in through the canopy, you might be able to see a couple of servos. I'm betting the one connected to the left elevon is either out of place, stripped, or jammed.

Unfortunately, that leaves you with few options. Horizon doesn't sell individual parts, only complete fuselages. The fuselage is solid, so there's no easy access to the guts. If there were, you could extract the faulty servo and repair or replace it.