The database [dbName] is not accessible. (ObjectExplorer)

Generally it is a bad idea to grant permissions directly to logins. You should create role objects in the database, and all permissions in the database should be assigned to the roles.

Then when moving the database to another computer (or reinstalling) the only things you have to modify are server logins and role assignments.

One possible problem is that you have two instances of the database, so you have to deactivate one instance, if you have your database opened and accessible in your server explorer in Visual studio close connexion then go to windows services and stop and restart SQL server service (MSSQL) then go to Management studio and open your database, it should be opened and you can explore tables from Management studio.

Login with sa and expand Security > Login, right click on the username and then properties, Click User Mapping and select the DB you want the user to access and then Ok