Hi,
I by mistake entered the master site domain name in place of slave site.
Due to this the contents of master site has gone and it became a new site.
What can be done to revert back this problem?
Thanks
Amar

You can either create another subdomain in your server that point to your master.
As this subdomain will not be in the list of slave site, it will be detected as the master website.

If you have a slave site where JMS is installed, you can try change the domain present in the JMS definition or just mark the slave site as "cancelled". So that it will be disabled.

If you can not create a subdomain in your server (solution 1) and not have any JMS in a slave (solution 2) then the lastest solution is to modify the files where JMS save the configuration to remove the entry manually.
You can try edit the /multisites/config_multisites.php file to remove the line of your slave site.
Becarefull that if you do that manually, you will probably be able to connect on your master but as soon as you will edit a slave site definition and resave it, this file will be rebuild. So the action will be to fix the slave site definition immediately to remove the mistake.
By security, make a copy of the file before to edit it.