I got a road bike through craig's list and I rode it for about 1.5 mile the day I got it (way too cold for a ride) and everything worked fine.

Yesterday, when I took it out for a ride, everything worked fine as long as I was shifting up to higher gears, but it won't change to lower gears.

There's only 1 lever for shifting (I don't remember how I got it to shift down but it had to be a pretty natural thing because I had not trouble going up and down on the test ride).

Before taking it out, I had it upside down while mounting a speed sensor. I do remember pushing the rear wheel so that the chain came off the gears (I don't know what you call the part that keeps tension on the gear set against the chain).

I though I may have put it back on the wrong gear. S pun the crank while it was over like that and the chain always ended up going to the same gear, so I figured it was sorted out.

The bike is a Jamis Satellite Sport, my first road bike. I've been riding for a year.

Any ideas or help welcome, thanks.

Rich