When a boss can't attack something most times he resets. A great example is Siegecrafter in SoO, where if you know that you are gonna wipe and everyone go on the belt the boss will reset since he can't hit anything. Hunters, mages, rogues and a few other classes can reset the boss if they're the last ones alive using abilities that make them untargetable by the boss.
I had knee surgery 3 years ago after I 'stubbed' my knee on the stairs while running up them. The doctor took my leg from the knee down, and then slept with my wife, shot my dog, ate my lunch while I sleeping in the hospital recovering and then found me after I was discharged and stole my crutch and beat me with it.
If none of the targets in combat are in the battlefield the boss will despawn or walk back to his normal spot, depending on the boss. very few bosses WALK back. Either way, it ends combat, and unless you ressed on a boss that doesn't despawn, you're then safe to ress and get away.