Is the ghost in your relations? If so, you can simply invite the sim to your lot and revive it. If not see if it is in manage households. If that also fails, your only option is to restore an older save where the sim has not died.
PS. since the sim did not die on your lot there will be no Urn/Grave.
I've had a Sim from a solo household killed in the jungle. Her best friend searched high and low for her ghost but couldn't find her anywhere. Many, many days later, her ghost just appeared at the Spice Festival. Her best friend moved her unhappy ghost into her Household and then built the skills and found the ingredients to create Ambrosia to revive her.
To this day it's literally the only time I've had a ghost appear at the Spice Festival. So even if you can't find them at first, they may still turn up in an unexpected place.
