How do you mourn at one of your dead sims graves because i cant find my sims grave

If the Sim died on your lot, the grave will usually be at or near the place where Grim came to collect them. If they died elsewhere, check your family inventory (they show up there when a Sim dies in a foreign neighborhood like World Adventures or University Life), or check the Mausoleum (Manage Dead) in town. Graves don't just disappear, but they're not always where you might think to look for them.