Bicycle Tutor

I recently acquired a Peugeot road bike. It needs a lot of work, but has basic functionality. The other day, I took the rear wheel off to transport it. When I extended the Derailleur to allow for the tire to come off, it kinda snapped back, out of it's spring position. When I put the wheel back on, the Derailleur was still in the wrong position. Not sure how to get it back. I am including a picture that illustrates the problem. I hope someone has some suggestions on how to fix it.

   

There's two coil springs in the derailleur. One on the upper pivot just below the axle and the other where the pulley cage rotates. You may have either broken one or caused it to slip out of it's mounting holes.
Look at the back (wheel side) of the derail to see if you can spot the spring and if it popped out. If needed, I believe you can takes these guys apart. You may be able to pop the spring back into the right place if it isn't broken.
If the spring is broken, you MAY be able to find a replacement, but tough. You can get a cheap replacement new derailleur easy from any shop. If you want to find one to maintain the classic look you can try ebay. Simplex derails run from a couple bucks to hundreds depending on condition and rarity.
Here's a good resource for classic road bikes:
http://www.classicrendezvous.com/ClassicRe.htm