Go to [Alfresco home]/tomcat/webapps. Verify that you have an "alfresco.war" file there and no "alfresco" directory. If there is an "alfresco" directory, move it out of that directory and try restarting Tomcat. If that doesn't work, try creating a new, empty directory named "alfresco" in tomcat/webapps and extract the contents of alfresco.war into that directory.
