First of all consider posting server logs
.In your case I believe you are trying to start it for first time.In your guest book project: simply put:
<threadsafe>true</threadsafe>
in your war/WEB-INF/appengine-web.xml
Like this:
<?xml version="1.0" encoding="UTF-8"?>
<appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
<application></application>
<version>1</version>
<threadsafe>true</threadsafe>
After running your app browse to localhost:8080
(or any other port in your case).You should see a guestbook page asking to sign in.
See answers Here.