Sehoo, I'm not sure if this will help but I have this problem now and again too. What I've found is that if I refresh my server page I'll notice my server actually stopped after a reboot. What I'm finding is that this is usually due to a bad mod. so i'll recheck the steam ID number or - in the case of mods that are older - I'll find they haven't been update in ages and cut that mod from my server. Once I do that and restart the server is usually find again. This actually just happened yesterday to me with the Improved Quality of Life mod. Once I removed it from my mod list my server booted and I was able to connect to it just fine.
Hope you can get connected! Those legendary chest are waiting for you!