This error occurs while I restore database from .bak file
A system assertion check has failed. Check the SQL Server error log for details. Typically, an assertion failure is caused by a software bug or data corruption. To check for database corruption, consider running DBCC CHECKDB. (Microsoft SQL Server, Error: 3624)
After this the Database goes offline and it property sets to Single User.
If SELECT @@VERSION < 10.50.1746.0 then it is possible that the issue you are coming across has been fixed in a specific cumulative update, and can be averted by updating your instance.
The particular issue is described here:
KB #980832 : FIX: "A system assertion check has failed" error message...
The cumulative update is here:
Cumulative Update #4 for SQL Server 2008 R2 RTM
But I recommend applying the most recent service pack and cumulative update available (since they come with a slew of other fixes as well):
SELECT @@VERSION? Have you runDBCC CHECKDBlike the error message suggests?