Z Axis Threaded Rod Not Turning

Hello all... still stumbling through getting to know this printer and how the entire process works. I've had a single successful print (in my eyes) in the short time I have had it.

So ... last night I had started a print and it failed. I then noticed that the left-hand threaded rod is not rotating with the motor. I understand that I need to tighten the "clamp" but prior to doing that.. is there some sort of procedure? I'm concerned about the carriage being out of level.

Any thoughts?

 

Just tighten the couplers, then manually level the bed with a piece of paper by hand for both left and right hand sides.

 

Thanks!