Checking my automated backup I realized that one of my database dump is empty. I chechked the database and it was ok. I tried a manual dump and I got this:
mysqldump: Couldn't execute 'Lock tables on `tablename`': Out of resources when opening file './databasename/tablename###.MYD'
After some googling I found a solution. I have the following line to my.cnf
open_files_limit = 4096
then restarted MySQL everything was ok.