What do I do with my old tank?

Hey, new member here (I used to browse the threads from time to time though). I have a bio cube which I believe is 15 gallons? So due to a living situation, I left it off and unattended (just water, sand, and live rock) for around 3 years with about 10% of the water. So my question is, what's the best way to go about getting my tank started back up? Should I just start from scratch? Should I ditch the live rock? Do I need new substrate? I appreciate all the help in advance, and if there's a thread for this already then please redirect me. I was unable to find something for a similar situation.