I am not sure what you are asking. A reboot usually fixes things like this. Shut down and boot up again. Assuming a reboot does not work try these.
If Paint.net is open then just close it and try to run your other programs.
If you are trying to open Paint.NET and it won't open, uninstall and reinstall. It is free and quick. Should only take a few minutes.
If you are trying to open another program and Paint.NET is not open, but it still gives you a Paint.NET error uninstall and reinstall Paint.NET.
If you uninstall Paint.NET and it is still not working you might have a virus and it is not really Paint.NET that is the problem. Update and run your virus scan.