Did you try to switch to attitude or manual mode once it started to drift away? If so, did it not respond?
There are some web sites that keep people up to speed of severe solar storms that can mess up the GPS systems.
It might have been one of those days.....
Give us the date and time of the incident and someone might be able to check that out for you.
nce the GPS lock is achieved, the FC knows where home is but can only return to it as long as the GPS constellation remain operational.
And if the GPS lock is not complete it never was able to establish the home point so it has no idea where to go.