Also, another solution would just be to use a vanilla minecraft.jar, which I usually do for servers. The modded one is for single player, which includes mods like BattleTower, and Multimine, all useless to SMP. It's just that on that day, my internet died earlier, so I switched to my modded one, and when it came back, I forgot to change, and that's basically where I went wrong in the first place.
On another note, I had this bad habit of using fly hacking to test my internet. Sometimes, when I think my internet has crashed, I fly up or around, and if I get teleported back to where I was, then I'm still connected. But if I can free-roam anywhere, that means I'm going to disconnect any time soon. Of course, this is if it had NoCheat.