In settings/control panel/power options there are ways and means of setting hibernation and standby. In any PC these may be over-ridden by lack of hardware capabilities (that is, your PC must have appropriate capability in the motherboard, hard drives, monitor....and video driver.
I'm only guessing here, but I suppose something was not right to cause the standby option to disappear, and now is available again due to new video driver software.
Probably Windows checks every boot-time that the hardware is responding as it should, which is one of the reasons Windows takes such a long time to start. If your hardware becomes faulty, or loose in a socket, then these checks would fail, so the standby settings change automatically.