What do you mean its gone? Have you typed /reload when rejoining the world? Oftentimes a message will appear for said datapack giving details like what version of the datapack you are using, as well as the creator(But not always).

Only other thing that comes to mind is maybe the datapack got disabled somehow. If this is the case, you can reenable it by using the following command:
/datapack enable DATAPACKNAMENOTE: Make sure cheats are enabled or the commands I have given won't work. You can do this by pausing the game with ESC -> Open to LAN, enabling the option "Allow Cheats", and starting the LAN world.

If none of that works, I need more information. Anything could have gone wrong, and simply saying that the datapack is gone doesn't give us much to work with.

• Did the datapack work before?
• Are you using the correct version of the game meant for said datapack?
• Perhaps there is some malicious software on your computer that is preventing it from working?
• Is the datapack still located in your worldsave? You can check by navigating to the select world screen in-game, clicking the correct world without logging in, and going to Edit -> Open World Folder, then using the window that appears, and going into the folder that says datapacks.
• Anything else you can think of mentioning that might help provide a clue as to what is going on?

I don't understand why that would happen... :(