This is often caused by the (accidental) removal of the JRE folder that is set in the Eclipse configuration. You can try following these instructions from the Eclipse wiki on how to configure the eclipse.ini file to include the the JRE location, or alternatively, launch eclipse from the command prompt using VM arguments. I have tried them both myself and in my opinion, the command prompt option works much better.
Once you are able to launch Eclipse, make sure you verify the installed JRE location under Java --> Installed JREs in the Preferences window.
