Is the tank new? You said a year old sand, wasn't sure if that was a year old tank, or just the sand you moved into a new tank.
How many fish?
Did you do anything new? Add fish, add rock? move rock around? Kick up sand?
If the tank isn't new, I would do a couple big water changes. If the tank is new, wait till the NH3 is gone, and then start doing water changes until nitrites is 0, and nitrates are low.
There are a lot of chemicals like Prime that can de toxify NH3.