When you plug a drive that's been formatted for Mac into a Windows PC, you may find that it isn't readable. This is because Mac uses the HFS+ format natively, whereas Windows uses ExFAT or NTFS. One simple way to make the drive readable on both platforms is to format it to ExFAT, which both operating systems can understand. This will erase all contents on the disk, however, so you'll need to transfer those files to a safe place first.
If you can't reformat the drive, there are programs available that will make it possible to read a HFS+ drive on a PC: