Here are a few.
1. You didn't deploy it at all.
2. You deployed it to the wrong Servlet container.
3. You deployed it but there was an error and you ignored it.
4. You deployed it successfully but at a different path.
There are plenty more.
Thanks for the reply, when I click run , I get the following information
OK - Undeployed application at context path /Hello
In-place deployment at C:\Test\Hello_code\build\web
Deployment is in progress...
OK - Deployed application at context path /Hello
Start is in progress...
OK - Started application at context path /Hello