Hi Sunil
There are many reasons that this can happen, the meaning of the message is that the client computer has lost connectivity to the server.
i have just finished fixing an issue with a client setup where this was corrected by loading the latest platform hotfixes, that has now resolved the issue, so i would suggest this as a first step
mbs.microsoft.com/.../NAV2009SP1PlatformHotfixOverview.htm
It can however be a problem with network connectivity as well so i would suggest you try the following as well, run a constant ping from the client PC's to the server and see if there are any lost pings, if there are the problem could be with your network connectivity
Cheers
Nev