have you ever played with mods on this file? Are you 100% sure you've found all the stardrops? (Did you revisit the statue in the woods and check it again?)
As a last resort, I can tell you how to edit your save file to fix it:
- Go to %appdata%/StardewValley/Saves and find the folder with your character's name or farm name on it.
- Open the folder and you'll find a file with the same name as the folder.
- Open the file in notepad.
- Ctrl+F search for "mailReceived"
- You'll find a tag that looks something like this: <mailReceived><string>button_tut_1</string><string>button_tut_2</string>...
- Add <string>CF_Mines</string> to the list of tags, so it look something like this: <mailReceived><string>CF_Mines</string><string>button_tut_1</string><string>button_tut_2</string>... (don't mess with any of the other string tags, they don't need to look exactly like this example. The only thing you need to do is insert <string>CF_Mines</string>)
- Save and reload your file, the game now recognizes that you received the stardrop from the mines
(Trivia: the reason the label for stardrops has "CF" at the beginning is because the stardrops used to be called "Cosmic Fruit" in early versions of the game)