To reinstall Windows Storage Server Operating System, download the proper version ISO below and burn to DVD. If you do not have an external DVD drive, you may use the program found at the following link to copy your installation to a USB flash drive.
http://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool
Your Windows Storage Server Operating System Version and license key is located on the top cover towards the back of your 3945N. Please be sure to download the proper Windows version for your license.
If you are missing your Windows version or license key, please contact Storageflex at 1-877-736-9407 and a representative will help you.
Download Windows Storage Server 2008 R1 64bit from here.
Download Windows Storage Server 2008 R2 64bit from here.
Download Windows Storage Server 2012 R1 64bit from here.
Download Windows Storage Server 2012 R2 64bit from here.