A sim died whilst I was playing the game.

@jayreok I must say.. I hate dealing with death and ghosts in the game. I'm a wimp... I almost always turn off aging, and that is one of the reasons why. you can turn aging off for your households &/or the community sims in your 'world' in settings: Go to the individual game (you can adjust aging per game) Options Menu > Game Options > Gameplay).

Anyway, it doesn't sound like you are enjoying them any more than I do, so I am including a link that will provide information about them, including how to get rid of them, depending on what you've decided to do about the ghost. http://www.carls-sims-4-guide.com/death/ghosts.php

Also, I did find a post by @Charliimai that had the following instructions, if the above did not have the information you need: https://forums.thesims.com/en_US/discussion/810464/ghost-wont-go-away

Enable testing cheats. (testingcheats true)
Enable CAS Full Edit (cas.fulleditmode)
Shift click to add the ghost to family.
Shift click and choose Modify in CAS
Click the ghost and an X will appear in the upper right hand corner of the thumbnail. Click the X to delete.
Note....this completely deletes the Sim from the game, so make sure you really, really, REALLY want the ghost gone.

Note... in case you need this info: How enter cheats: https://www.ea.com/games/the-sims/the-sims-4/pc/how-to-cheat