If half the roots are still in the ground, I'd say dig under the other side a bit so that you can "tip" the plant back upright, letting the exposed root ball go back into the soil. Then stake it and I'd say it has a good chance of recovering completely. If there is new growth, that's a good sign already.

Thanks, Bill. I went over today and did some clean up over there and there is new growth so that's good. I'll try to stake it up and see what happens...

No staking necessary. If there is growth you can cut it down to 6 to 12 inches. It might make it easier to wrestle it back into place and not have any weight to pull it back over before it has time to settle back in. Cutting butterfly bushes down in the Spring after the appearance of new growth is a standard procedure and nothing you need to worry about being drastic. It keeps the blooms at a reasonable height.

Well that certainly would make things easier! Probably a stupid question - but how much new growth will it put back on this year if I whack it back? Losing that height will leave a pretty big hole - but it's better than having it keep coming down until it gets reestablished. I doubt it's ever been pruned back (like I said, these people could care less about the yard.) Yesterday I filled 3 lawn bags and didn't make a dent. I'm estimating 4 or 5 more - more if I whack back this bush. haha

Cutting it back won't stunt the growth, but the root damage may set it back a bit. I cut mine back every spring and they bounce right back to 6 to 8 feet.