Happened to me yesterday.. Several reasons why it can happen.
1: Upgrade of hardware or software causes Service host to take it off line
2: Denial of Service Attack, by criminals, either on the forums themselves or on the service host provider.
3: Other internet problems including but not lmitied to domain name server issues (indlucing DNS attacks) and service node attacks
And yesterday, in my case, a vehicle attacking a telephone pole somewhere (Also took out cable TV and Electricity)
Had a car once slide off an icy road and through a big white painted metal box..... Yup, the master telephone (Modem) feed to my ISP. Instant OFF LINE.