HI there,
well that is very interesting.... I got a 2002 147 late last year, owner had just had the selespeed actuator replaced ($3500 AU!!). A month after I got it, the selespeed failed, and I was left sitting at the lights with no gears available, much like you I'm afraid. The problem is that you're likely out of selespeed fluid, probably caused by a leak in the actuator. There are other possible causes, hopefully cheaper!!, but my guess is that it's the actuator, sorry.
The odd (and annoying) thing is this.... For the past few weeks, I've also been getting rear fog light failure warnings!!
AND in a service check today, I'm now informed that my actuator has again developed a small leak!!!!
So it's back to the original company to get it sorted, hopefully more permanently this time....
Sigh.... So good luck with your diagnosis, selespeed is brilliant most of the time, but expensive if something does go wrong... Just go somewhere reputable and brace yourself.
Cheers
TiggerK