Thanks for your help.
I have deleted the mu-plugins directory and renamed the plugins file as mu-plugins .
All the plugins were disappeared and my sites Started working well.To get the plugins again I again changed the name to "plugins" .
I will activate the plugins again. But do i need to maintain both the folders mu-plugins and plugins ?
I have created other multisite and in that too the default plugins folder is plugins . And I have created mu-plugins for domain mapping plugin?Am I need to use both the folders for ever? Is there any chance for conflicting of plugins in the future?