Drive D became active, no longer boots

Drive C ran out of space, so I used the Disk Manager to make Drive D the active drive thinking this would let the computer use the space there. Instead, the computer no longer boots up like it used to, says Bootmgr is missing. I have tried repairing the boot startup, to no avail. How can I make Drive C active again if I can't even get windows started to get to the Disk Manager?!

Drive C ran out of space, so I used the Disk Manager to make Drive D the active drive thinking this would let the computer use the space there. Instead, the computer no longer boots up like it used to, says Bootmgr is missing. I have tried repairing the boot startup, to no avail. How can I make Drive C active again if I can't even get windows started to get to the Disk Manager?!