First make sure to double-check, as chances are that the missing objects are just hard to spot. Here's what you can do:
1. Hide buildings in Edit Mode to see if the missing object is just hiding behind a taller construction (to do so, click Edit Mode and select the transparent building icon).
2. Open the store to select the missing building. If it's still in your town, you'll see it right in the center of the screen.
3. In case of missing decorations, check the tab with the decorations you have stored.

If the object is actually missing and you can't find it, please contact our customer support and provide the following information:
- What object is missing.
- What you were doing before it went missing.