Originally posted by Jool I just don't understand why it would stop working and then work again. Any ideas?
I've not had this experience, so this is just a guess, but ...
In mechanics, there is something called "stiction" (check the wikipedia entry for some more info), wherein it takes more force to get something starting to move than is necessary to keep it moving once it has broken free. I can imagine something like this occurring in a lens that hasn't been used in a while: any lubricants in there could congeal a bit, and you can even have bonding over time between pieces of metal that are in contact with each other. (I can not get some steel setscrews in my jointer loose from the aluminum hub that they are screwed into!! There has been some kind of bonding between the metals over several years.)
The solutions mentioned above: trying a different camera (there may be more power sent to the motor) and manual focusing (breaks the stiction, thanks to the extra force your hand can apply) seem consistent with this.