Young wizards have a few routes of recourse when they've lost a wand.
They can in fact Accio it, if they can find a sufficient focus object that is wand-shaped (or perhaps any object, as long as it 'works'), or get a friend to Accio it for them. Wands (other than perhaps the Elder Wand) are not a listed exception to Accio.
If for some reason there is something preventing it from being Accio'd, or if a wizard can't find a proper focus, they can always retrieve it with clever application of another spell. As Richard pointed out, Luminos works even when you aren't holding the wand, so it would at least allow a wizard to see where it is.
And, if all else fails and the wizard simply can't find their wand (or worse, it has been destroyed) they can always get another one made, though it's liable to be expensive, and may not be as well-suited for their use as their original wand. Although, in both Ron and Nevill's cases, they turned out to be much better off with new, non-inherited wands.
So those are their three courses of action: accio with a friend or a makeshift focus, use a different spell to locate it, or simply get a new wand made.
Or they could just use their hands to pick it up, but what kind of wizard would do that?