When I try to talk to her, I just hear a clicking sound and nothing happens. I can still enter the 'command mode' by holding down E, so I can tell her to wait or pick up stuff from the ground, but I'd still like to get this fixed. All other NPCs are fine, it's just Lydia.

I've no idea what brought this on. I have some mods installed but everything has worked fine so far, this is the only problem I have.