Which version of Lightroom are you using?
With Lr closed go to the location where your Lightroom catalog resides(By default it is Pictures/Lightroom)
Delete the .Lock file with the same name as your catalog.For instance in the Picture below it is Lightroom4 catalog.lrat.lock
In your case it will be Lightroomcatalog.lrcat.lock.Delete it
Let us know if that helps
Hello so I am using Lightroom CC 2015
I switched my user to another (In my pc) and the Lightroom works there but not on the this username
This is Permissions issue with that user account.
Restart Your Computer .Follow steps mentioned Unable to reopen catalog in Lightroom
Grant Read/Write Access to all Adobe Folders and Pictures Folder
Best option would be to Run SFC-scan https://support.microsoft.com/en-us/kb/929833