The first thing I would recommend would be to check the properties (right-click on the mailbox store object in Exchange System Manager and verify the database path location. Is it where you expect/want the database files to be?And, is there actually enough free space in the volume?
On the same topic, if you have been running tests (MS Loadsim initializations or runs, for example) you may have run out of space on your logs volume. This situation will also result in a dismount of all mailbox stores within an Exchange storage group.
With regard to transaction logs, check the space available for the path specified under the "General" tab of the properties of the storage group ("default first storage group"). In a non-production environment, the stores are generally not backed up. Thus, the transaction logs are not truncated and fill the available space on the volume. If you find your logs volume is out of space, you can enable circular logging (not recommended for a production mailbox environment), or run a backup of Exchange, which will truncate the logs and should then permit you to mount the store.
If there is enough free space in database and logs volumes, retry the store mount and then take a look at the Exchange server's application and system event logs for any errors/warnings around the time you retried the mount for troubleshooting hints.
Next, I would suggest checking the store database for consistency, as a torn or otherwise broken database will cause mount problems.
Microsoft has a utility for this verification. More information about what this utility does and how to run it is available here.
If you're still having problems, please let me know and we can dive a bit deeper into your environment.
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