How can I connect to a database after its disk has come back online? I have problems with database after reconnecting a hard disk:


.Net application works with database.
Disconnect hard disk with physical files.
Make any request to the database (of course we will have an exception, Unable to open the physical file).
Reconnect hard disk.
Any query to the database in the future will still throw an exception, Unable to open the physical file.


How can I recover connection with physical files without restarting the SQL Server service?
First, tell your customer to get a real disk. Using a remote disk this way is exactly why you are having difficulty at this stage, and it could very well lead to a corrupt / unrecoverable database. You are rolling the dice every time this drive goes offline, even if only temporarily.
Next, check the status of the database:

USE master;
GO
SELECT state_desc 
  FROM sys.databases
  WHERE name = N'your_database';


If this if OFFLINE, and you're sure the drive is available, run:

ALTER DATABASE your_database SET ONLINE;


Now, this might not work - as I suggested the database could come up SUSPECT or it might not come online at all.
If / when it does come back online, take a full backup. Schedule these (and/or log backups if in full recovery) to run much more frequently, to cover your butt.
Tell your customer to get a real disk. This may work in the short term but this is a ticking time bomb.