What you will have missed from the context there is that the dragon did not come alive again, as such, as become undead. There is a subtle distinction. These characters who look like the bad fúckers from the third Lord of The Rings movie are a zombie army of undead, or wights as they are known, and their corpses are animated by a cadre of generals known as the White Walkers, lead by a supreme badass known as the Night King (the dude on the horse dispensing the anti-dragon artillery).
When a corpse is re-animated as a wight in control of the White Walkers, it's eyes open with an icy blue glow and the feckers are incredibly hard to kill and keep being brought back to life anyway. Losing one of her three dragons was bad enough for Daenarys, the
Khaleesi, Queen of the Andals and Ruler of the Seven Kingdoms known to some as 'the blonde chick', but having one now under the control of the enemy and practically invincible is a Big Problem.
The weakness of this zombie army, as we discovered earlier in last night's epidode, is that if you take out a White Walker (only possible with special weapons) this may cause the wights under their control to basically dissolve. Maybe if you take out the Night King himself, the whole lot will melt, as you will remember that Sauron's army did in LOTR when the source of his power was destroyed. So basically they should hire a couple of hobbits to sneak up and stab him with a dragonglass blade.
In case you were wondering when nine blokes (nine, eh, that might be familiar) were taking on a mighty fùcking army, their plan (bit mad really) was to go beyond The Wall (Scotland basically, but worse) and bring back a wight alive in order to persude all the human factions warring among themselves that they should unite against the common enemy. As plans go it paid a high price.
Hope this helps