Hi,   Based on your description, you're trying to add D drive to C drive with the video that you have watched right? The video is only for combine D and E drives which and should not be related to the OS C drive. You may have corrupted the OS C drive, and the data cannot be recovered.   For server's configuration, combining available space is not done at OS level but in hardware level, which need to be done on the RAID controller of the server. 
Hello, today I was attempting to combine my D volume harddrive with my C. In this process I had to remove a volume such as the D drive, after that I attempted to combine with the C, it told me to refresh as the Disk Manager was behind I did it also said if that didn't work restart the PC. I restarted and got a message no Operating System found. I didn't clear the C drive whatso ever as it didn't even let me combine the drives, I had some data on the server not backed up and I am quite depressed about the thought of loosing the data. I am a noob when it comes to harddrives and linking them so I might have done something wrong.   Default OS: MS Windows 2019 DataCenter Server Server: Dell PowerEdge T410 (Yes I know thats an older one but it does what I need it to do or did what I needed it to do)   Please, please let me know if you know the issue here or how to solve it, you would bring alot more happiness to me and my friends who host a minecraft server on it.   
Removed a disk volume (D) and the server will not boot to an OS anymore and 
says no OS found upon boot.