Welcome to our NS Community, heymarian_. You should definitely give them a call on 030 - 75 15 155. If you explain your situation and ask if something's possible, then they can apply leniency.

Sitting in the first class without firstclass-ticket is not allowed. Sometimes the customerservice will change the correction fare. You need to contact bij Phone, chat, X or Facebook.

Can i still call them even though it's been weeks past?

Yes, that's no problem! It's even preferred after a week, because then the chance is that the trip has been loaded correctly into the system.

I just wonder why you didnt notice the change in the color of the seats, the 1 and the 2 on the doors and also the class stated on your ticket. There are also announcements on display in English and Dutch that its not allowed. Theres also numbers outside before getting in the train. 

If you have a subscription, like Flex you should definately know how to use the trains over here.

Im sorry, im just baffled people taking public transport in countries and not having the responsability to find out the rules. Its not like they dont have 1st and 2nd class anywhere else..