Nissan Micra dci sport+, not starting

[Gilesy]

Its been standing for 3 years, when i pump the pumpy thingy (my knowledge is not good) it starts, and continues to run, someone has said the fuel is going backwards, told it could be the pump, tank, or seal round the injectors? where do i start to try and resolve please, and any idea on prices? I have just returned to work after being unwell for three years - any help much appreciated, also told it could be letting in air somehow, is it worth fixing? the car is a 55 plate, and in excellent condition otherwise