Welcome back!
The water will almost certainly need to be replaced; I'd not trust it to be ready to sustain life if it were me. New salt water will give you new nutrients, etc. that the tank will need.
The sand
might be okay to keep, but it would be a risk. Again; were it me, I'd replace the sand. Or, if you want to reduce costs, remove what you have and go without sand entirely ("bare bottom"). This will limit your livestock selection somewhat, so that may not be an option depending upon what you want in the tank (can't really have a sand-sifting goby without sand, eh?).
The rock most certainly does
not need to be thrown away. There are several options here;
1) Simplest - Deal with the water and sand, turn on a heater and a pump to move the water about. Add some ammonia (or just add a raw shrimp) - then wait. You'll very likely go through a new cycle as the bacteria in/on the rock comes back to active in the presence of food. Once the cycle finishes, you can move into re-adding livestock. While this is the easiest, it's also the most likely to have you dealing with challenges for the next year+ of the tank (undesired algae, bacteria blooms, nutrient imbalance, etc.)
2) Middle Ground - Take out the rock a piece at a time. Scrub it with a good brush to clean off any detritus, algae, etc. and set it out to dry for a week or so (it'll stink). Scrub it again to remove any dried stuff. At this point, you'll have "dry live rock" that you can use in the tank reboot. You
could possibly use the rock without new bacteria, but it would take longer than if you were to use some bacteria-in-a-bottle, for sure. This method will reduce the problems you will likely face, but there will still very likely be some.
3) Most work - Remove the rock and put it into a bleach solution (BRS has
a good video on this process) to "sanitize" it of all life. This will be a total reset of your tank - it would eliminate all bacteria, algae, critters, etc. Once the rock is cleaned, dechlorinated, and cured, you can re-add it to the tank. Add some bacteria-in-a-bottle and some ammonia (or shrimp) and you can re-cycle the tank. This will result in a totally clean tank, but is obviously the most work and time-consuming.
If you have the time and inclination (and feel comfortable with the bleach work), then I'd use option #3, myself. If you're going to restart a tank, may as well give yourself the best chance of success. And odds are slim that you have enough life in the rock to justify keeping it in a "live rock" state, I'd guess. My guess may be wrong, however, so really any of the options above could get your tank back online.
