Is my lease still valid if my apartment complex office burned down and now they are requesting I sign a new agreement?

So long as the unit that you were renting did not burn down the lease that you have for the unit would still be valid and legally binding.

If your rental burned down or was so severely damaged that it could not be occupied then under the doctrine of "frustration of purpose" your lease would be voidable at the option of either the landlord or the tenant and legally unenforceable.