It's a known issue, if you're mining and the server crashed or is reset, you start out as if you hadn't mined anything at all, unless you docked somewhere with the ore on your ship. When you dock, your ship and cargo are saved to the server. If you haven't docked and didn't log out via F1, the server didn't save anything. Hence, empty cargo hold.
You're not going to get your ore back, better just get back to mining.