Reset a community space back to original?

If you want the community space to be a regular community lot, you'll have to open build mode in that lot and change the lot type to another lot.

Otherwise, I don't understand the question well. If the options are public garden, marketplace, or maker space, then the lot should be in default state. Right now it is not possible to add more lot types for community space to change into.

You probably don't need to do anything. Your community space should already be the default community space if that's the choice you don't get.

Are you wanting the original build back?

Start a new game, go to that lot, save the lot to your library. Go to your actual game, and place the library lot there.
- That will put back the original build, and all of it's settings.

Skyrim, where every other mod fatally kills your game save, taught me to always keep a 'clean save' around, and to save often. I've yet to have a fatal issue in Sims 4 though - over in Skyrim you install one mod to change a hair color and you can be sure you will get fatal exceptions every 3 seconds...

But I have needed that clean save many times already in Sims 4. I've done a wonderfully amazing 'best build ever' on a lot, played for a few hours and then looked at what I did and thought... no... what fool thought that was a good idea... and so used the clean save to clear it out.