Urn gravestone disappeared missing

@Mrs_Bloody_NL I play with aging off and have not dealt with death, but I found a thread with this information that you could try:

Go into build mode and open the cheat bar (ctrl shift C) and type in bb.showhiddenobjects. Then exit and enter build mode again. There should be a grave on the lot, most likely inside another grave (I think they mean here if you had a different sim die before). You can also turn on cheats and shift +click on a grave to add ghost to family.

There was another advisor that said:

So you do "testingcheats true" cheat and add the dead body/ghost to a current living family and then you type in "bb.showhiddenobjects" in the cheat bar and buy the urn. When you place the urn in the world you click on the ghost that's now apart of your family and have them click on the urn and click "posses urn." They will then go into the urn and you can place the urn in your inventory. I put the urn in my inventory and brought it to the grave yard I made and placed it there. The urn turned into a tombstone when placed in the grave yard and I just made a little grave with flowers for them and saved the property. Done. Now the home they were in is empty and they're in one place with all the other ghost sims. 

So... perhaps using cheats to show hidden objects may help you find the urn they get the dead sim back into it, etc., depending on what you want to do with the urn/ghost.

Plus there is information about death, etc. here that looks good: https://simsvip.com/2014/10/06/sims-4-ghost-guide/