1. Are you using Mods? If so, make sure your mod versions are all the same.
2. Could you find it before but can't find it now?
Yes -> mod/version issue, see #1
No -> have you added it as a favorite in your server list? If no, go to your main steam window, click "View" (top left), Servers, Favorites and then add your server with the following format IP Address:Port Number (like this 216.244.93.102:27315). Once you do this, launch ark and you can find your server easily under your favorites.