Same thing happened to me with my right hand. It happened right after I started riding a road bike with drop handlebars & combo brake levers/shifters. Previously, I was riding a Terry Classic w/a flat handlebar (really had wrist numbness w/that set up, but haven't experienced it w/the drop handlebars). Anyway, I put in a lot of miles when I first switched to the road bike and I think my hand/forearm simply got tired. I had a hard time holding a pen/writing, and a couple of fingers on my hand would shake. Creepy beyond belief. Anyway, I stretched out my forearm/wrist good and made sure I rested that hand as much as possible. I've also made it a point to relax my hands as much as possible when riding, flex my fingers, and move my hands around from hoods to flats. Luckily, I never completely lost strength like you did, as I could still ride/shift with no problems. It was post-ride that it really affected me. After taking a day off, stretching, and resting, I was good as new. Your poor hand just wasn't used to shifting/braking for all those miles By the way, check out the Castelli Donna gloves that TE sells. I love them!!!