since.2006  

The connection to adb is down, and a severe error has occured.
You must restart adb and Eclipse.
Please ensure that adb is correctly located at "xxx" and can be executed.

ADT突然无法调试，如果试了其它方法无法解决，查看下任务管理器中是否有tadt.exe这个进程，有的话杀掉重启ADT试试。:-)

新版本QQ中加入了Android设备连接电脑提醒功能，启动了自己的一个adt.exe造成ADT无法调试，害得俺找了N久才找到问题原因。