Oh, that's a bit strange. Let's try the easy option first: Does it show up if you press B in-game? That's the default keybind to toggle the number on and off.
If that doesn't fix it, I'd like to know a bit more:
What version of the mod and Minecraft are you using?
What other mods, if any, are you using? And if you're using Mod Menu, does Show Durability show up there?
Can you send a Minecraft log file? Doesn't have to be anything specific; just a log file from a session where you start the game and load into a world should be fine.
I am able to launch Minecraft with it open but it doesn't show the durability for the tool on the hot bar. I don't know how to fix this.