Insufficient data.

Was "straight south east" just a continuation of the direction it had been heading or a new heading?

Battery was full? If you never found it you doon't know trhat. It may have been recentrly charged but do you meter it at the field before flights?

Antenna extended? Did you range check it before the flight that day?

Forget to put the arm screw in a servo?

I take it ALL control commands did not work. That could mean the battery connection was bad, bad switch (it happens), out of range (What receiver? Some park fliers are only good for 600 feet or so), a bad bad crystal (I know some flyers who will not fly a once-crashed crystal).

Had one a a local club where the pilot tried to trim the elevator and instead switched off the transmitter. Ooops.