If you reformat a hard drive you'll wipe all of the device-specific drivers. While Windows includes loads of generic drivers it can't include every possible one for every possible computer. So you might need to reinstall the network adapter driver. To do so, you'll need to use your other computer to download the driver installation file (from the manufacturer's website) to a USB memory stick, then transfer that stick to the duff computer, copy the file to it, run it and restart the computer.