score:2
This is pretty much the standard, and it prevents hidden city ticketing, among other things - most airlines apply the rule that if you don't take any leg - be it leg A, B, or C of any flight - any legs after that one will be cancelled, and you'll likely lose your money for them too. It's almost certainly in your terms of carriage for the ticket.
At this point, your best bet is to read your specific airline's terms very closely to see if they mention this. If not, ask them about it, and if it does, well then you will have to ask them very nicely about it and explain why they should 'break' their rules to allow you the leg.
Good luck!