the analysis was working on version 10, after migration to version 11, it started giving error
[nQSError: 14025] No fact table exists at the requested level of detail
11g is more strict that 10g was in terms of a conformed data model. These types of errors almost always stem from something in the BMM being set up incorrectly, so I would start there. There are several things it could be.
Check your levels on the LTS being used. Set any levels to total for things you want the LTS to work with, but that it does not join to. This will force OBIEE to ignore that item in the join criteria, since there is no join. Set these levels on the column as well.
If you have levels set to detail, make sure the physical join actually exists.
Run a consistency check, and look for any warnings where it says no physical join, or logical tables source joins table at incorrect grain (I can't remember the exact wording, but you will know it when you see it).