The computer is not recognizing the device because the appropriate software or driver is not installed on the device.
Most of the time when you connect a smartphone into a compatible laptop the device drivers or software automatically install, so the laptop may recognize the smartphone.
Here are some possibilities as to why the device is not recognized.
1) The USB port is bad.
2) The USB port is not activated
3) One of the devices does not support the other
Here are a few possible troubleshooting solutions:
1) Go to the smartphone manufactures website and download and install the updated device driver and software then plug in the device to see if it is recognized.
2) (if Windows OS) Go to the Device Manager to see if the device is showing up when it's plugged in. If you see the device in device manager right click the device and choose to update the driver.
3) (if Windows OS) Additionally, in device manager you can check your device's USB ports to see if there is an issue with the ports that may be resulting in the smartphone not being recognized. You can choose to re-enable the USB ports or update the software for the USB ports.
Hopes that helps resolve your issues.
Open Device Manager Windows Help