Well Poop
 

So, out on my first ride of the year, and first longer than 30 second ride on a new old bike (new to me, 70 something Schwann conti) everything is going all happy hunky dory then when I go to shift it suddenly becomes hard to peddle. I get off my bike an it looks like my chain jumped off one of the jockey wheels and my derailleur decided to wedge itself under the cassette. Along with this, both the cables for the front and rear derailleur have gone somewhat slack so...yay. Maybe i should stop with gears and go make it a single speed? Or have someone do it for me lol

Or an IGH. more gear ratios , but without a derailleur to mess up.

Is there a question here?

If so photos of the rear der in the condition as it was when you got off the bike would help. i can think of a few possibilities but without more data can't say for sure what happened.

BTW the rear cogs are most likely a freewheel, not a cassette. Andy.

Try this Park Tool Co. » ParkTool Blog » Rear Derailler Adjustments (derailleur) and this Park Tool Co. » ParkTool Blog » Front Derailleur Adjustments to get you started about your cables problems .

i don't think there is a question, the OP is probably just trying to spread the misery around to lessen the pain :). i'm sure he'll be up and running again in no time.

Chains rarely jump off the idler pulleys. More likely the derailleur was not properly adjusted or was previously bent inward in a fall, and when behind the FREEWHEEL when you shifted to the large cog. Lots of potential for damage.

It may just need to be "dialed in." I've had that happen a bunch of times on "thrift store" bikes I am bad to get. After a few rides, I tend to get that bug out by making little more than adjustments.

Bummer, man. You gotta do a full inspection and test everything on a "new to you" bike BEFORE you ride it.