Go to the virtual machine's configuration > Hardware > Network > Source > Shared Network (Recommended) option should be set by default.If this helped and internet connection restored, enable all services back one-by-one to identify the problematic 3rd party application. NOTE: in step 3 keep both Parallels Tools services enabled. Perform clean boot in Windows by disabling 3rd party applications' services including 3rd party antivirus software which may have a negative impact on a network driver's installation.Before going any further please take a snapshot ( Actions > Take Snapshot) to revert to a pre-troubleshooting point.Make sure your Mac is connected to the internet, Windows will not be able to access the internet if it's disconnected or inaccessible on the Mac side.

Virtual machine's settings misconfiguration.3rd party applications or antivirus software are blocking network connection.Network settings misconfiguration in Windows.Other home network computers are not reachable.Issues with network dependent software even though the internet in Windows is working fine.Internet connection is low or unstable.No internet connection in Windows, however internet is accessible on the Mac side.