The above issue occurred to me once I did a force shut down on my machine where I had MS SQL Server Manager working .Although I could query for the databases and tables, when I try to expand a database in the object explorer it gave me above error. There could be couple of reasons for this and error log should give you a hint what exactly caused the problem.
You can read more about what exactly msdb is from official page here. And there is an important mark as “After any operation that updates msdb, such as backing up or restoring any database, we recommend that you back up msdb. “ which I missed to do and must have caused above.
Surfing through the internet provides you various methods of fixing starting from rebuilding system database in SQL server 2008 to reinstalling the application.
What instantly fixed my issue was to replace existing
files in C:\Program Files\Microsoft SQL Server\MSSQL10_50.SQLEXPRESS\MSSQL\DATA. I got these 2 files copied from another working machine, Stopped the SQL service running in my machine, removed the above existing 2 files from their location and added the new 2 copied. Once I restarted the service , issues has been fixed.