Can't shift down - no resistance when trying to do so

[attack plan r]

I'm having a bit of a problem and as a newbie to bike maintenance, I don't know where to start.

Today, as I was trying to shift to an easier gear on the back, I was met with no resistance from the lever. The lever to shift to a harder gear works just fine but the other one does absolutely nothing, at any gear.

This seems like an easy fix but as I said, I'm pretty clueless. I don't know what to search for because I don't even really know what my problem is. I have Ultegra components if this helps.

(My first post here, I'll make a proper introduction later)