今天笔记本电脑忘了插电源线，出去一趟后回来发现没电关机了。重启后发现网连不上，提示“网络禁用”，上网搜了下：
http://ubuntuforums.org/showthread.php?t=1505217
这里给出了原因：
Have you had a failed hibernate that locked up and you had to do a hard reboot?
Apparently, the problem is that the Network Manager marks network interfaces disabled during the hibernate, and re-enables them on suspend. When hibernate fails and the user has to reboot, the second step never executes, and the interfaces remain disabled, as far as NM is concerned.
See http://osdir.com/ml/debian-bugs-dist.../msg07864.html (or LaunchPad bug #524565) for details and a fix (which worked for me):Code:service network-manager stop rm /var/lib/NetworkManager/NetworkManager.state service network-manager start
大概原因是网络管理器在电脑没电休眠时禁用网络，然后挂起时会重新启动它，可是因为我在电脑没电休眠的时候直接强行重启电脑后，第二步没有执行，所以系统设置里总是处于停止状态。执行上面红色字体的命令后就可以了。感谢万能的google.