Resurrect a Sim

Try to reload an earlier save file. https://sims4.crinrict.com/eng/2014/09/recover-backup-save-game/

Reloading, as Gagea says, will work if you have an earlier save file to reload to. ...Have you looked under things in the kitchen? I know that sounds dumb, but I've had glitches where my urns and other things accidentally spawned under countertops or appliances (usually when I've been building and placing decor with move objects on and forgotten to turn it off again before going to live mode.)

One of these things :
http://cdn2.bigcommerce.com/server5400/tglr4/products/851/images/1610/6011_1__91723.1378303172.320.320.jpg?c=2

@StuffyMarx ROFL!!

One other question though, is it common for sims to get heat exhaustion inside their house?? I've not been playing w/seasons all that long yet.

Well, I've played this household in the current save for about 4+ days now and so far, no tombstone and no ghost. This is sure looking more like a glitch than anything else. I do still have the original save and am thinking about going back to it.

Sorry about your suddenly deceased sim @Sonshine77 . Yes, they can die of heat exhaustion anywhere if it is hot and they're not wearing hot weather clothing or there's not AC on. You get an Uncomfortable moodlet with a sun on it for a while to warn you ahead of time.
To avoid this, you can change into hot weather clothing when it's hot, or place a thermostat on your lot and set it to cooler when it's hot outside.

For the tombstone, it may be hidden by another object on your lot, or perhaps even under a floor if your foundation is raised. You might have to move things around to find it.

Do you have another Sim in the house? Check their inventory and also the house inventory.

I did look under everything and also below the foundation. There just is no tombstone at all. Also no ghost. Where do I find the thermostat for the house? I've looked everywhere, but cannot find it. I looked in "outdoor activities" (suggested by somebody), but it is not there. I'm not sure why it would be in that category though.

One more thought about the urn, do you have any other urns or tombstones on the lot? New urns and tombstones like to occupy the same spaces as existing ones sometimes.

For the thermostat either Appliances > Miscellaneous, Electronics > Miscellaneous, or type thermostat in the search box.