[Elecraft] K3 Suddenly no power out / high SWR on 10m only

[iain macdonnell - N6ML]

So I was happily running (100w) in the 10m RTTY contest, when suddenly
my stream of callers ground to a halt. I looked at the meter, and saw
that I wasn't making any (significant) power any more (~1w). The K3 is
reporting high SWR, but an external meter is not. All other bands are
transmitting fine (incl. 6m). The antenna is OK (hears well, and
another transmitter sees normal SWR on it). What would cause high SWR
/ no power out on 10m only?

TIA,

    ~iain / N6ML