我的问题是：

9:15:53 AM  [Apache] Error: Apache shutdown unexpectedly.
9:15:53 AM  [Apache] This may be due to a blocked port, missing dependencies, 
9:15:53 AM  [Apache] improper privileges, a crash, or a shutdown by another method.
9:15:53 AM  [Apache] Press the Logs button to view error logs and check
9:15:53 AM  [Apache] the Windows Event Viewer for more clues
9:15:53 AM  [Apache] If you need more help, copy and post this
9:15:53 AM  [Apache] entire log window on the forums

根据log查找解决方案，

1．80端口冲突，解决方法：

打开目录C:\xampp\apache\conf(我的安装目录为C:\xampp)下的httpd.conf文件，将Listen 80和 ServerName localhost:80中的80改为8081或其它值（尽量不要使用0－1023端口，这个是系统保留端口），要保持一致。
2．监听端口443冲突，解决方法：
打开目录C:\xampp\apache\conf\extra (我的安装目录为C:\xampp)下的httpd-ssl.conf文件，将Listen 443这句注释掉：# Listen 443或将443改为4433或其它值（尽量不要使用0－1023）。

再修改了以上两个端口后，再次启动apache，it works！

9:18:03 AM  [Apache] Problem detected!
9:18:03 AM  [Apache] Port 80 in use by "D:\Program Files\Skype\Phone\Skype.exe" with PID 4808!
9:18:03 AM  [Apache] Apache WILL NOT start without the configured ports free!
9:18:03 AM  [Apache] You need to uninstall/disable/reconfigure the blocking application
9:18:03 AM  [Apache] or reconfigure Apache and the Control Panel to listen on a different port
9:18:03 AM  [Apache] Problem detected!
9:18:03 AM  [Apache] Port 443 in use by "D:\Program Files\Skype\Phone\Skype.exe" with PID 4808!
9:18:03 AM  [Apache] Apache WILL NOT start without the configured ports free!
9:18:03 AM  [Apache] You need to uninstall/disable/reconfigure the blocking application
9:18:03 AM  [Apache] or reconfigure Apache and the Control Panel to listen on a different port
9:18:03 AM  [Apache] Attempting to start Apache app...
9:18:03 AM  [Apache] Status change detected: running