Well, title says it all. While on my latest travels through Skyrim, I entered the city of Whiterun, and the tree was, well, gone.
Not invisible, but missing. I could go right through where the tree once stood, and the console isn't detecting anything around there. It's an entirely empty space.
I am using no mods, and have all the DLC installed, plus I have completed the quest "the blessing of nature", where I chose to get the Gildergreen to bloom again.

If anyone has a fix for it, I'd be much obliged.

Never heard of that happening. I'm no expert on what goes on for the PC, but I suppose reloading an earlier save is out of the question?

Well, I tried that, but every time I enter the city, it's still gone...

What way did you restore the tree? There's 2 ways and one is known to be rather glitchy, while the other is a safe bet.

I took the sap to revive the tree, not the sapling.
But that was like.... 40 hours ago.

Well you did it the safe way, the tree should be fine. I have no clue what to tell you. And you say your not running any mods, weird I have never heard of this bug/glitch

Well, as long as nothing else disappears I shoul be fine. I gues...

This has been reported in the unofficial patch thread in the Mods Forum. The Gildergreen returns (briefly) if you save in front of its location then load that save.

The issue will be addressed in the next release.