You could try Unrelenting Force to see if you could knock it down but it could just make it worse though, maybe Telekinesis but aiming would be hard if you can't see the target. If your on the PC use the console to spawn one.
Unrelenting force could work. I've used fireball and ice storm so far and they both don't do anything. I can't reload to an earlier point because of how much I have done since then. I was hoping that when i turned it off and started another quest and came back to it, it would be in the tomb. Wasn't so i'm still stuck. Its on XBOX.
I had to solve this by bringing up the console in PC, clicking on the coffin and then typing "disable" and hitting enter. This makes the coffin disappear and you can take the sword. I couldn't reach it otherwise, it was stuck behind the coffin. If you feel guilty afterwards you can bring up the console again and type "enable". Good luck!