So I picked up a 2006 ZX14 from a dude and he said he was riding and down shifting and all of a sudden his back wheel locked up and the bike wouldn’t go in any other gear. Bike won’t roll with clutch in so everything is pretty much locked up. Just wondering what this might be and where to start looking?

I'd start by pulling the clutch cover and checking the clutch, shift star and linkage.
Does the clutch lever move at all ? Shift lever move at all ?

Mad

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I'd start with removing the oil cap and smell for oil or has a burnt metallic tinge to it. Next is to remove the right side crank cover so you can turn the crank. Crank shows it can move either direction, says it's not the engine. Next would be to pull the clutch assembly off and see if the shift linkage spring or arm has moved away from turning the shift drum.

Once the clutch assembly is removed, the star plate bolted to the shift drum's end, see if you can spin the star with the 6mm bolt that holds the star to the drum. Does not move, going to split the cases then. These moves are all short of pulling the engine.

Then, pipes off, oil pan dropped to see if the engine took a shit with brass and aluminum in the pan, or a gear tooth sitting in the pan.

The smell test knows for sure if it's a waste of time with a stuck crank.

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Clutch goes in all the way. Shift lever moves up and down.

Also the bike cranks and runs great.

So I picked up a 2006 ZX14

I'm going to assume you bought a bike that ran and you rode away with it. Or did you tow it home?

and he said he was riding and down shifting and all of a sudden his back wheel locked up and the bike wouldn’t go in any other gear.

No doubt the bike can leave from a dead stop and move away in any gear. So you rode it home in a single year, now you're here.

Bike won’t roll with clutch in so everything is pretty much locked up.

To separate the clutch from the back wheel is to keep bumping the bike back and forth with clutch lever in until the plates break loose.

Also the bike cranks and runs great.

So you can pull the clutch in and the bike does break the clutch loose from the back wheel.

Clutch goes in all the way. Shift lever moves up and down.

With the oil cap off, you can watch the pressure plate move out. Shift lever moves up and down yes, but can you shift down?

Or are you wondering why you can't hand shift the lever thru the gears? So bottom line, did you buy a bike stuck in gear?

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