Please do these steps to troubleshoot your issue.
In eclipse go to Windows>Preferences>Server>Run Time Environments.
Now please check following things
- make sure apache tomcat 7 is there.
- If this is present make sure that the path specified in "Tomcat installation directory" there is still a tomcat at that path.
- If above two are good then delete the runtime and re add your tomcat as runtimes.
I hope this helps you fix your problem :).