Well lets see here yup yup, your data is gone. I'm guessing your external hard drive was NTFS/FAT32 formatted. Time Machine requires HFS+ file system in order to backup correctly. If the HD that is plugged in does not match these requirements and Time Machine is asked to use the drive, it will reformat it to the HFS+ file system. Windows isn't made to access this type of drive format so if you want to bring it back to normal(your data is still gone) you can follow the steps in a few seconds. I do want to point out that while your data is gone, you can get programs like File Scavenger (highly recommend) to scan the drive and have a chance at restoring the lost data. The demo of File Scavenger allows you to do full scans but you can only restore up to 32kb files or something like that, so you will have to get the full version or find another program. Anyhow...
After you plug in the drive to your Windows machine, you can right click My Computer on the desktop and select manage. Expand the + next to Storage and then click Disk Management. You should see a few volumes listed and from there you should be able to format the drive. If you have questions on any of this just speak up and I'll try the best I can to help ya.
