Hi
It sounds like something has gone wrong with the permissions in SQL server ot the user account on the services, it could also be that you have set permissions in NAV or SQL that are now being enforced, this is not something that you can easily fix if you dont know SQL or NAV
2 Options, perhaps contact your NAV partner to assist
Alternatively you could try uninstalling NAV and SQL server express, then re-installing, this may be a quick fix