Before the War in Heaven the Necrontyr were already incredibly advanced civilization and they sent their colonists out into space on mighty Torch Ships. Now lets say one of those ships survived the 60+ million years of drifting through space. Now it contains the Necrontyr how they once were before the Biotransference damned their species into a soulless existence. And the oldest of Necron worlds are near the Koronous Expanse and the Halo Stars. So now the crew is in whatever system is needed when that incredibly old Torch Ship drifts into the system after millions of years. What would the Necrons do to get it back? I would want to tie it into their quest to become flesh and blood once more.

First you need a reason why the ship isn't well outside the galaxy…

Such a ship powers to 0.99c (or whatever) cruises (most of the journey) and then decelerates for the final leg. If the drive failed, when did it fail? It would have to has failed at a relatively slow velocity to still be in the galaxy. Perhaps the ship now in some wide galactic orbit? But why did it fail? Are the colonists in stasis or is it a generational craft?