Check in Disk Management as it may just need a drive letter assigning to it:
Control Panel > Admin Tools > Computer Management > Disk Management
Look at all the horizontal ribbons in the bottom-right panel. If your Iomega drive is shown there (it will have no name, no letter, and may be labelled "RAW"), right-click it and choose "Change Drive Letter & Paths".
Click "New"
Choose a letter.
Click 'OK'
If it's not listed in Disk Management it typically means the drive or it's converter-board has failed, and no recovery software will help you under those circumstances. You can try removing the drive and buying a third-party external enclosure for it, or send it to a data recovery specialist firm (but that is very expensive).