I have done some investigation into this matter and have not found any obvious cause in terms of any recent code changes.
I created a new library and let MC discover my videos in several different locations, including mapped drives. After MC imported the videos, some files ended up having no duration. This is caused by MC failing to build a DirectShow filter for that file type during import. When a bad file is encountered subsequent importing of other files of the same type would also fail. I had a large number of avi files failing, which turns out was due to a couple of them being bad (unplayable).
I also had all my mkv failing, until I reinstalled CCCP and did a "Re-register filters" in CCCP configuration. For some reason my DirectShow filters had been badly messed up.
So, here are a couple of general advices on this matter:
1. Make sure your DirectShow filters are in good order. If you use CCCP, which we recommand, you can reset its settings:
Start > All Programs > Combined Community Codec Pack > Settings, on the second page of the wizard, you can check "Reset all settings" and "Re-Register Filters" checkboxes.
That usually will fix a lot of your DirectShow filter problems.
2. If you have a large number of video files that have no duration after import, you should first restart MC, then select those that do not have duration and choose Library Tools > Update Library (from tags). This should fix most of them. If that did not work, and
assuming you have DirectShow filters in good order and most files of the same type do play, you should try identifying those files that would not play and remove them from the library, then repeat this step.