Dismounted Exchange server 2010 database and couldn't mount it.

Hello all,

one of the database mailbox was dismounted and i couldn't be mounted. see the error log below.
"Error:
Couldn't mount the database that you specified. specified database: database;
Error code:an active Manager operation failed. Error: the database action failed. Error: operation
failed with message: mapiExceptionCallFailed:unable to mount database.(hr=0x80004005,ec=-1021)"

how can fix this issue.

regards