The Crossbow Expert feat states that as a bonus action, you can "attack with a loaded hand crossbow you are holding."
However, nowhere in the rules does it specify a "loaded" versus "unloaded" state for any weapon. There is a "Loading" property (PHB p.147) that just says that "because of the time to load this weapon, you can fire only one piece of ammunition from it..." but doesn't specify an amount of time, type of action, or anything to load the weapon. (And, in any case, if you have Crossbow Expert you can ignore the "Loading" property.)
Under "Ammunition" (PHB p.146) it just says that "drawing the ammunition from a quiver, case, or other container is part of the attack" which seems to imply that crossbows are not kept loaded all the time, but are loaded and fired as part of the same attack. So is there some other action that needs to be performed to load it (so you can have it loaded first, before the trigger hits?)
[The specific situation that I have in mind was the case of a character with Crssbow Expert and two hand crossbows, one in each hand: Can he fire both crossbows every turn, using his normal and bonus actions respectively, even though he doesn't have a hand free to load the crossbows? Do you need a hand free to load a crossbow?]