Somehow I had a system crash, after which I was not able to reopen the session I had in KDevelop, I just get an error dialogue saying that it is already open in another running instance. So i was looking at how to rectify it.
So i found out that this is in order to restrict opening many instances of KDevelop. So it create a lock file for the system to realize that we already have started.
One solution to this is to reboot your system (Atleast it worked for me).
Another solution i found out after searching is to delete this file
where ~ is the path to your user.
Or, simply run the following command:
sudo rm ~/.kde/share/apps/kdevelop/sessions/*/lock