Mongo DB Can't Start We have had a mongodb running fine for a few weeks and all of sudden it has gone down.

We are unable to start it back up as it is complaining that the dbpath (/data/db/) does not exist. I've tried routing through the documentation and have seen others suggest using the dbpath flag, however still no luck.

We are starting it with sudo service mongodb start and the config file dbpath config points to a directory with our db in.

Any advice would be greatly appreciated.
Since you are running out of space for the journal files, here is what the docs says:
journal

journal Default: (on 64-bit systems) true
Default: (on 32-bit systems) false
Set to true to enable operation journaling to ensure write durability
and data consistency.
Set to false to prevent the overhead of journaling in situations where
durability is not required. To reduce the impact of the journaling on
disk usage, you can leave journal enabled, and set smallfiles to true
to reduce the size of the data and journal files.
Note You must use nojournal to disable journaling on 64-bit systems.

journalCommitInterval

Default: 100 or 30
Set this value to specify the maximum amount of time for mongod to
allow between journal operations. Lower values increase the durability
of the journal, at the possible expense of disk performance.
The default journal commit interval is 100 milliseconds if a single
block device (e.g. physical volume, RAID device, or LVM volume)
contains both the journal and the data files.
If different block devices provide the journal and data files the
default journal commit interval is 30 milliseconds.
This option accepts values between 2 and 300 milliseconds.
To force mongod to commit to the journal more frequently, you can
specify j:true. When a write operation with j:true is pending, mongod
will reduce journalCommitInterval to a third of the set value.

For more clarification, please read Recover MongoDB Data following Unexpected Shutdown. You may need to run validate() on your data collections to verify integrity.
You should also read about Journaling
SUGGESTIONS

Looking back at this phrase If different block devices provide the journal and data files the default journal commit interval is 30 milliseconds.
You should make store the MongoDB journals on a different disk from the Data.
You may also want to shrink the journaling files if you haven't already done so.
You should see if the journal files are RAM-based. If they are and if you ran out of memory, that may explain your crash.
See Documentation on --smallfiles