In the same folder as your Lightroom catalog file is a file whose name ends with .LOCK
Delete the .LOCK file
Do not delete anything else.
Could you please attach a screen shot of the error that you are getting?
Are you getting the message " Switch to, retry or cancel"?
Also which operating system are you working on, Windows or MAC?
Try restarting your computer once.
Hello TanjuHere is the screen shot
Thanks for your help
Trying again re screenshot