Cannot connect to the COM3 port. Possible reasons are another application is connected to the port or the port does not exist.

When I use the following code:

delete(instrfindall); clear s

s= serial('COM3'); fopen(s);

I get this error message:

??? Cannot connect to the COM3 port. Possible reasons are another application is connected to the port or the port does not exist.

Error in ==> fopen(s); % open the serial port for writing and reading

Yes my device is connected to the port but I want to use the incoming values from the same device so what should I be doing in that case ?