If it's an inactivity issue (in other words, Liferay stops working after two hours of inactivity), then it's most likely a database connection pool problem. If your database connection pool is not properly configured to test connections before returning them to the applications that request them, that would be your problem.
In order to resolve your issue, you'd need to update your database connection pool settings in your Tomcat configuration (or, if you're using the Liferay ones provided in portal-ext.properties, the ones there) in order to make sure that connections are tested.
