I just had a thought (It happens sometimes!), since Boris FX bought out GenArts, if you Cannot get a response back from GenArts, I would suggest you go to this website on Boris FX to open a case (what they call a ticket): https://borisfx.com/support/open-a-case/
Under Product, you may have to fudge a bit, at least to get the request sent out since it appears they do not list Particleillusion yet on that list.
Hopefully Boris FX can reset your activation back again. Hopefully GenArts only had a "Glitch" and did not get your original response...
That Is strange since GenArts indicated users are supposed to use that Website. Can you go here to see what GenArts is seeing: http://www.genarts.com/sn-lookup.html and type in your serial number. It should show you what system GenArt thinks Particleillusion is connected to.
Here is what I show the process of getting Assistance.
Then I would also suggest you send an email to genarts at support dot com to let them know you also sent them a request for assistance via their website. I really hope that Boris FX gets involved as well since it seems strange that Genarts is dropping the ball here.