Database error MongoDB: disappeared database
The error you are getting suggests that the data is still there, but something is causing a problem when it mmaps the data files. This line is the first thing you need to look at:
Thu Jul 26 12:05:44 [conn5] Assertion: 13636:file /var/lib/mongodb/baza.4 open/create failed in createPrivateMap (look in log for more information)Check the permissions on that file (and other files in the data path), make sure nothing is wrong from that perspective.
Next, there is the out of memory error:
Thu Jul 26 12:05:44 [conn5] ERROR: mmap private failed with out of memory. (64 bit build)
You might be running into this bug: https://jira.mongodb.org/browse/SERVER-6414
So, you can try 2.0.7 which contains a fix for this, the release candidate is available on the downloads page as of writing this.
Finally, if you have the option you may be able to solve this by increasing memory on this instance (if it is a virtual machine of some sort) 1.4GB is not a lot of RAM in terms of running a MongoDB instance.