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 'D:\Tool\Android_setup\adt-bundle-windows-x86-20130917\sdk\platform-tools\adb.exe' and can be executed.

解决方法：

adb kill-server
adb start-server

      -- //可能需要先关闭Eclipse，执行完命令后再打开Eclipse      

又遇到问题;

adb server is out of date.  killing...
ADB server didn't ACK
* failed to start daemon *

解决方法：端口可能被占用
C:\Users\Administrator>adb nodaemon server
cannot bind 'tcp:5037'               -- //端口被占用
C:\Users\Administrator>netstat -ano | findstr "5037"
 TCP    127.0.0.1:5037         0.0.0.0:0              LISTENING       12220  

                                -- //pid为12220的进程占用了5037端口
C:\Users\Administrator>taskkill /f /pid 12220
成功: 已终止 PID 为 12220 的进程。  

          -- //打开任务管理器不一定看得到占用5037端口的进程，推荐命令行方式结束进程