What exactly are you trying to achieve here?

Why would you try accessing a resource URL on the site when the database is down?

Are you trying to edit the file or just read it?

Yes i'm trying to read some files. 

It's still not clear to me what you are trying to do.

If the database is down, you shouldn't even be able to browse your site (http://localproject.dev) in a web browser (you'd get a connection string error).

If you're just trying to access the Demo.css file to open it in an editor to view or edit it, just use Windows Explorer and browse to the local path on your machine.

The file should be located at [yourProjectRoot]/Templates/Demo/Styles/Demo.css, and you can open it in Notepad or whatever.

I want to show a custom error page and on custom error page i have to show site logo and want to apply css which is existed in Site folders. so i'm unable to access http://localproject.dev/Templates/Demo/Css/Style.css file. how can i read the files ?

OK, so your custom error page is unable to read the css file.
Since you're getting a redirect loop error, it's most likely a access rights problem. 

Your custom error page is likely a page you want anonymous users to be able to see, so the "Everyone" group needs to have read access to it.
You can set this via the "Set access rights" feature in EPiServer (available in edit mode by right-clicking a page, or via the Tools menu in Admin mode).

Also, since your custom error page reads from the /Templates/Demo/Css/ path, you need to make sure "Everyone" can read that path on your disk. In Windows Explorer, right-click your Templates/demo/css folder and check if Everyone has read access. If not - add it. 

You are right. i'm getting "Since you're getting a redirect loop error" error. i have check the access right for the folder & even i add IIUser & give full access to it. but still i'm getting redirect loop error.