When you power off your droplet it is simply powered off. Your disk space and data are still there and are preserved.
Keep in mind that you are still billed for droplets that are powered off because your disk space, CPU, RAM, and IP address are all reserved while it is powered off.
If you want to save a Droplet for future use take a snapshot of it and then destroy the droplet. You’ll be able to create a new Droplet from the snapshot image anytime to bring it back online but you would lose the IP address and get a new one. The snapshots cost $0.05 per GB per month.
Hope that this helps!