My mobile not connected laptop by usb cable

What kind of connection? Are you talking just connecting the phone to the laptop so you can access files on the phone, put files on the phone or power the phone? That the USB cable should do. Unless there is something wrong with the cable, the phone's port or the USB port on the computer.

Now if that isn't what you are trying to do, and say you are trying to tether the computer to the phone, that is a whole different story. Not all phones are capable of that.

So what are you trying to do exactly?