I am currently working with Hibernate and Sql Server 2008. The question is if the webapp is running and I am modifying the data in the tables of the database (tables that Hibernate mapped to objects in my webapp) will that influence Hibernate in any way ?
Will it crash ?
IF you modify only the data hibernate will not crush but it's possible that your modified data to be overrwritten if same objects (rows) were loaded into you webapps. If you modify your database structure then it is possible to receive errors for the modified table (for example if you delete a table or a column mapped in hibernate; adding tables, columns etc. is ok).
Don't know much about Hibernate, but I suspect you might have problems with transaction locking or cached data if you're updating the DB in the background while your Hibernate application is running as well. You'd probably need to test the various scenarios to make sure things work the way you want/expect them to.