Acrobat x, inept customer service, theft

So i purchased acrobat  x many years ago. Its been installed on my computer for that long. Last month i suddenly received an invalid serial number error, then the program stopped working.  I dont have an adobe account,  and i will not engage in a subscription (would rather use another company program). 

Adobe employees give me nothing but a run around.  They keep saying the serial number is not legit,  when it is. When i send documents of the purchase  they conveniently say they never received... 

I paid for this program, fairly, and now i cant use it.  This is theft folks!!. I require a serial number that works. Please advise,  TY, DW