Hello,    You most likely have wither Standard or Datacenter edition of Windows. The .ISO linked below should be what you need to perform an OS repair.   [URL]   When you boot to the OS media, you'll be able to get a command prompt, because it won't be connected to your old data. However, if you don't know the system's password, I can really only recommend reinstalling. The product key would be needed for this, though. Repairing without reinstalling shouldn't require you to know the password.
The office I worked in had a network rack with various PCs, networking firewalls, and a PowerEdge R420 server but there is no one left in the company that knows the password to the server PC. I have looked around and identified I need to boot it from a USB for repair because I don't have the installation CD/DVD but I am not sure which ISO file I need. The Windows EvalCenter ([URL] has a few options but I am not certain which one I have. The options from the Eval Center are Server 2012 R2, Server 2012 R2 Essentials, Hyper-V Server 2012 R2, Windows Server 2012, and Hyper-V Server 2012. I've also run across sites referencing Datacenter, Standard, Foundation, and Essentials but I have not been able to dig around in the settings, Boot menu, or UEFI menu to give me any confidence. Upon boot up, my screen displays "Windows Server 2012 R2." Can anyone help direct me to the right ISO? And one bonus question for anyone familiar, if I don't have the product key (because I can't log in to run command prompt) will I still be able to run the evaluation ISO file and get back to the retail version of 2012?
PowerEdge R420 Windows Server 2012 R2 OS repair - Which ISO file?