IF the rim looks intact but it just has a significant "hop"/wobble to it, you can probably true it up fine by carefully tightening the spokes while balancing out the lateral pull on the rim. If, on the other hand, the rim appears to have been dinged or dented by blunt force, that may explain how the spokes got so loose and you may have trouble getting it trued up sufficiently.
I had a bike on my rear rack when I got rear-ended a few years back. The frame and crank died; most everything else was salvageable, but the rear wheel was messed up pretty badly, too. I took it to a shop with an old-timey owner (Erlton Bikes in Cherry Hill, NJ) and dude had it trued up in no time; the wheel has held up since, too. So, yeah, even if there was trauma to the rim, it's worth a shot. The impact totaled my Honda, but the Sun Rhyno-Lite/Shimano XT/Wheelsmith rear wheel was salvageable. Go figure.