If your computer does not recognize the external hard drive, you should first check whether you are using the correct USB port and whether a sufficient power supply is ensured.
Most external hard drives have a USB 3.0 interface, which you should connect to an appropriate PC port. Often it helps if you simply use a different USB slot and/or reboot your PC. If available, you can also try a different USB cable, as some USB cables are only meant for charging and do not allow data transfer.
We also recommend unplugging unneeded USB devices from your computer to free up as many USB resources in the system as possible, as each USB controller in the system can only run a limited number of devices.
Depending on the hard drive, you may also need to connect it not only to the PC via USB, but also to the power supply via a power adapter. Make sure that you use the correct power supply and that it provides the hard drive with sufficient power.