I was trying to build my game and realized I had done it wrong, so I pressed cancel and all of my game objects were deleted. Is there any way to retrieve them?
Are they really gone from disk (and not in the recycling bin, either)?
If so, simply type git status to see which ones were deleted, and then git checkout -- <name> in order to restore them (or use SourceTree).
If you aren’t using version control on your project, then consider this a lesson in the school of hard knocks.
Is it possible you were placing objects while in Play Mode? Any objects placed while in Play Mode will vanish when you return to Edit Mode. (You can, however, select those placed objects while still in Play Mode, Edit->Copy, return to Edit Mode, Edit->Paste. This is particularly handy when you want to keep GameObjects created by a script at runtime as pre-runtime placed objects.)
Anyway, this is a really easy mistake to make. (There’s a preference setting to change the main color of the GUI to remind you you’re still in Play Mode.)
That’s my best guess, since your actual files are intact. 