I first did the side quest to retrieve an item for some shopowners. I cleared the entire place of enemies, including the Boss at the end. But I didn't take the dragonstone with. At that time I didn't have room for a heavy zero valued item in my bag. So I left it there. Now I need it for a quest and I can't find the body of the Boss. How can I solve the problem without a restart?

console it in if you are on the PC. Check the chest again, build up your alteration and learn the detect dead spell (Alteration 75) and try to locate the body. Pretty much your only options. I'm impressed by your looting choices though. Fifteen ancient nord axes, check. Seven iron swords, check. Mysterious Dragonstone...bah that's junk. heh

I'm not sure there is any other way. In the future I'd advise keeping anything that looks unique, dropping anything else instead. If you pick up quest items, they should weigh nothing anyway, even though the item will indicate a weight.

lol I love sarcasm!

Uhm, those were actually my first thoughts as well. How can you just pass on a strange Dragonstone?

But, yea, you have to either find the Dragonstone ID and use the console or load an earlier save. I checked around the web, there are no other solutions. Some people say the stone is on a table, on the ground or in a chest. Just search the whole chamber. Although, I do remember finding the stone on the boss' body. Still, can't hurt to look for it around the room.

You pick up a stone, it has weight 25 value 0. I am atm weighing 299/300. I just figured junk. But I was like, if it's a quest item it won't disappear. And it's not like you have much place to store the thing. it's a giantic burden to run around with ...
I play on Xbox360. If I played it on a PC I would have solved by now

It's a quest item; if you picked it up, you shouldn't have been able to drop it. Check your inventory under Misc, maybe it's in there and you didn't realize it.

Quest items dont actually weigh anything. Even though it says it does.

When I did that quest, I was ready for the draugr to get out of his crypt and smacked him down immediately. It almost went through the floor leaving just enough for me to click on to loot it. I think the floor in that area is not too solid. If I plan on returning to a place to get more loot, I will try using a chest to put in what I can't carry. I guess you learned something from this.

After a long search I managed to find the dudd and retrieve the stone.

PS: the weight did count as I tested it.