what to do

Your granddaughter will contact you as she gets older, you had a close relationship so at 14 she will make her mind up and they won't be able to stop her

No way of knowing who is in the wrong here without more info, but it is unfair to stop a GD seeing a grandparent she has spent so much time with, can you stay in contact with her via WhatsApp or email until her parents hopefully come to their senses. I imagine she is missing you.

From the scant information given, that just sounds vindictive of your d-i-l.
Hope she chooses to keep in touch with you via social media or email.

There is nothing you can actively do.
Just sit tight and wait, and hopefully access will be allowed again.

I cannot believe many 14 year olds would actually go along with this, do they live close to you, does she attend school close to you? Unless there’s some form of court order in place there’s zero stopping you from phoning her or contacting her directly and meeting up with her in town.

Ah just read this, so your ex DiL wanted you to tell her where her ex husband, your son, is living?

Does your son have access to your granddaughter? If so then visit her there.

Also, you did exactly the right thing not to disclose your son’s address to them for any reason, without his consent or knowledge.