Is it trying to access a resource that's not available, like a NAS?
If nothing else works you could remove the library so it has to start over.
Note that you will be starting with an empty library.
If you have a backup you can restore from that. You will have to reenter your license.
Do Go->Utilities->Terminal
then at the prompt (without MC running)
mv ~/Library/Application\ Support/J\ River/Media\ Center\ 26 ~/Library/Application\ Support/J\ River/Media\ Center\ 26.bak
Then restart MC.