Server not listed?

Yesterday my friend and I, were playing on my DarkRP server. We were both connected, and so were 2 other random people. I then left the server on over night and came back to see that the server was offline. I then turned it on knowing that the server process may have just crashed, left it on all day, came back from school and I was connected fine. My friends were trying to connect, and the server did not appear on their list. Even when I sent them an invite via Steam, it said “Server is not responding.” I have checked to see if I have portforwarded correctly and everything. Please help?