Sql-server – Suddenly cannot access SQL Server database

This is a known issue which has several causes:

1. The first thing to check is your permissions on local machine:
2. Are You a local Administrator?
3. Is there a group policy that might prevent you from installing \ debugging?

If these do not resolve the problem in your case, and you have verified the required permissions, please provide more info:

1. What parameters do you use during installation (beside the default ones)?
2. Which Edition of SQL Server are You installing?
3. Do You use Default instance name?
4. On which O/S are You installing?

EDIT:
Next steps: 1. open the folder ":\Program Files\Microsoft SQL Server\100\Setup Bootstrap\Log
2. Find a file named Detail.txt
3. This file has a complete log, You need to find the FIRST error that occured during installation.
4. Paste it here, only the part with the error.

EDIT:
Added Solution to comments, waiting to hear if the problem is solved

Well, suppose you have a command to execute your backup like this

BACKUP DATABASE [naass] 
TO  DISK = N'C:\program files (x86)\Microsoft SQL Server\MSSQL.3\MSSQL\Backup\Naass.bak'' 
WITH NOFORMAT, INIT,  
NAME = N'naas-Complete Database Backup', 
SKIP, NOREWIND, NOUNLOAD,  STATS = 10

this will fail if the process that executes this command doesn't have the permissions to write in the mentioned folder. And Windows 7 (for very good security reasons) keeps the C:\program files folders and its subfolders write locked also to an administrator account.

You could mess with the default permissions and give to your folder above write permissions for your account but of course this is not the recommended way.
Much better, simply change the script to work on a different directory on your disks.

BACKUP DATABASE [naass] 
TO  DISK = N'D:\BackupSQL\naass.bak' 
WITH NOFORMAT, INIT,  
NAME = N'naas-Complete Database Backup'
SKIP, NOREWIND, NOUNLOAD,  STATS = 10

Of course this new directory should have the correct permissions.