I am using MS SQL Management Studio 2012 to restore the database from a
.bak file. (That file is from the backup of the database on server)
In my new database, the triggers are missing. Why did I lose the triggers?
Sounds like the backup was created before the triggers were put into the database. Make sure that the backup is current. SQL Server won't just drop the triggers when it restores the database backup.
A wild guess - are the triggers based on CLR code? If CLR Integration has not been enabled on the machine you are restoring to then the triggers won't work.
CLR Integration can be enabled with the following code.