If you can access other databased from Sql Server Management Studio and have issue with one or two databases and getting error of The database is not accessible (ObjectExplorer) try out following steps
This is generally happens when you restore database from other server. It may happen you have just restored the database of that server not specific user and trying to access that database.
This can be caused when you have removed your default database of particular user and you are trying to login with that credentials.
1. You are logged in with the user who has permission to access that database. Try to login with sa user or Windows Authenticated User.
2. If database is not accessible even with sa or Windows Login User try to bring it offline database and then bring it online and then check.
3. Last Option you can deattach the database and attach it again. (It may be possible data loss here – 99% it won’t be). Just be careful while doing this.