Well, it's probably not what you think. Perhaps you could take a photograph and post it here?
The usual hydraulic line on a mountain bike is a connection between the hand lever for the brake and the brake caliper, which squeezes around the disc rotor. It is the rotor that is connected to your wheel (via bolts surrounding one side of the hub). The disc slip within a narrow slot in the brake caliper, which is mounted by bolts onto your front fork.
So, when you release the quick release lever (and maybe continue to back off the opposing nut a bit), the wheel should be free to move out from its slot and drop from the fork. You will see the rotor disc slip out of its caliper slot as the wheel drops, too.
When you put the wheel back on, make certain to guide the rotor disc back into its caliper slot, and then the quick release lever and hub back into its fork end. Adjust axle nut so that lever tightness begins to make contact with the fork when the lever is in pointing straight away from the bike. It should clamp firmly with finger pressure to close fully. It it is too easy to fully close, then the nut needs a turn or two more to tighten. If it's too difficult to fully close the lever, then the nut needs to back away a turn or two.
Before tightening the lever fully, make certain that the disc rotor is centered within the brake caliper. You can check this by peering into the slot, maybe with a small light held behind it, or something white as a background, so that you can see the tiny gap on either side of the disc rotor.
Hope that this helps you.
