The best way is to reinstall the disk that came with the product. It may be an old version but it will work on the apple device you connect it too. You can then update to new version online.
Make sure that you checked everywhere on your hardrive. Make sure you check your C; Drive. If you really dont have it on your computer then you need to reinstall it. If you had songs that you had purchased then they should be saved on your membership. I hope that this has been helpul.
You will need to scan your computer for malware and or, viruses or trojans. They are the sole reason for deletion of certain programs and so forth, as they can take over a computer and do whatever it pleases. You can just reinstall it and it should be there, but check your control panel to see if it was removed.
The easiest solution would be to first ensure it isn't in Control Panel -> Add/Remove Programs (Or programs & features on Vista/7), then try re-downloading it from Apple's site.