Check your paperwork, did you sign anything?
Were you warned (paperwork or vocally) about what would happen if you gave a deposit and then could not get finance.
I'd ask for written reasons as to why they have not returned your deposit in full so you know where the problem lies, if there is one and can take it from there.
Could you not get any finance or just not one which was suitable for you, rates, repayments etc...
Could you apply for finance with another company or take a comparative bank loan, only consider this if you can afford repayments though!
What kind of place is it? Is it a large national dealership or small private place?