If you have the Product Key you can reinstall with any retail installer: Windows 7 Universal Installation Disc - Create
If you've lost the Product Key you can audit off the HD using Belarc Advisor if it's still installed.
Why would you buy an OS which is yours for life, can migrate to any other machine of your choice and never expires, and not want a hard copy DVD for reinstallations or even repairs? Why would this and the Product Key not be guarded as you would any other valuable?
Because it was cheap, perhaps the Student Deal?
If you bought the Student Deal, MS made good on their fulfiller Digital River not providing a DVD or even an ISO here: Microsoft Fixes Windows 7 Student Edition Upgrade Problems