Thanks for the reply.

Yesterday it happened... SQL was not allowed any new connections..but it was showing 99 % CPU usage by SQL.

we restarted the server ... and server is back. I found the error "LazyWriter: warning<c/> no free buffers found"

I think the Memory is not sufficient for the Environment or else we need to lock the memory by assigning the Lock pages in memory user right to the user account that is used as the startup account of the SQL Server service.I will try if it happened next time.. or we will increase the RAM

Thanks.