Will I lose all my data?

I have a DS223J NAS with a HDD that I used for a while. For various reasons, I had to unplug it and leave it disconnected for a couple of months. I received a mail that my Quick Connect ID was about to expire, but I couldn’t do anything about it because I didn’t have a way to plug it in. Now I’ve plugged it to both LAN and the electrical plug, and it opened as if that was the first time I turned it on. I had to reinstall the OS, and now it’s asking me for new credentials. If I continue, will I lose all the data that I had put on it before? What do I have to do in order to have it up and running again without losing data?