Citroen Saxo Starting problem (immobiliser?) help needed?

Brought a citroen Saxo 1.4 2002 reg today. tried fitting a new radio and the battery light came on. i drove to the shop and disconnected the radio. i then reconnected the radio when i got back in and went to pull off. i then turned my headlights on and the car just stopped running and i havent been able to start it since.. I can get lights on the dash (handbrake, battery, airbag and oil) but only intermitantley. other times i jut get the battery light flashing and a clicking noise coming from the passenger side dashboard. really have no clue on whats happened. thanks for ANY advice.