Note to reader: I use cheats for this game
I used the 'trigger age transition' tool on my elderly sim, and it killed her and then her urn just lay there. Although the urn was just laying there, it disappeared randomly. Where did her urn go?
Check your inventory. (Family & Personal).
If it's not there, perhaps another sim stole it.
It is entirely possible that your use of cheats may have something to do with it, especially because cheats and cracked games are notorious for being full to the brim with bugs.
If it isn't in your inventory, and you can't find it on another Sim, it may have been relocated to a totally random spot, (common in the X-box 360 version's cracks,) or the code you used to cheat it forward, if that is indeed what you did, may have had a bug in it that "forgot" the urn was there.
You will often find that the finished, polished way the producers create a product so advanced as the Sims can be completely undermined by the cheaper code that hackers/crackers use.
Lesson: Don't crack your game. It makes more problems than fun in the end. Mods are better.