Car will not start or even turn over, nothing after lay up, new battery but no dash lights showing p

 I have just bought a Mazda MX-5 1600cc Mk1 J  Reg 1991 with A/C , P/S , E/W , E/L , The previous owner drove the car into his garage before going abroad to work three months ago it was running perfectly OK . On coming home he went to start the car but the  Battery was dead . He went and bought a brand new Battery but did say when refitting the last terminal there was a flash . Upon turning the ignition on no dash lights plus nothing else works only the retractable headlights  . We have turned the engine by hand so thats not seized the Battery Leads are all OK . Any one got any ideas please . Cheers Tony