@cleo00
The population cap was mentioned in another post and it is thousands - so I don't think that is the reason, assuming your sims do die? :)
Files can become corrupt if you use mods or custom content; sometimes even if you don't.
I remember having a legacy family that lived in Neighbourhood 4 (Legacy) and I played this family for about 5 generations.
I took a back up of the game and moved the EA folder to another location and generated a complete new Sims2 game folder which I enjoyed starting fresh again with Pleasantview etc.
After a while though, I thought I would return to my legacy family, so copied and pasted Neighbourhood 4 to this new EA Folder.
Graves in Pleasantview (Goth) and Strangetown (Specter) disappeared! :(
I wished I had taken a back up of my new game before I added in Neighbourhood 4 (Legacy) but I guess there are lots of stories like this.