It's a bug that's popped up before. I've had it where only teammates could access it, and I've had it follow me around the map before. This was very early after release. You might search in the Bugs subforum.

If you were wearing the Megingjord and lost it when you died, your weight capacity has dropped. You may not be able to “open” the stone because you cannot simply absorb 100% of the items there. Try opening your inventory on it instead and taking select items first.

Originally posted by Lenny Rat:

If you were wearing the Megingjord and lost it when you died, your weight capacity has dropped. You may not be able to “open” the stone because you cannot simply absorb 100% of the items there. Try opening your inventory on it instead and taking select items first.

I don't think it's because of the weight, you just become encumbered untill you re-equip the megingjord, I think OP means he can't access the tombstone interface at all