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I see that you've written that you live in the UK. I am not sure about the law there, but here in the US, grandparents have legal visitation rights to their grandchildren.
You may want to give your daughter a little while to cool off -- something is bothering her, and she may just need time to work that out. Maybe her partner can tell you what she has said.
Sometimes, in therapy, people seem to get worse before they get better-- it's a natural part of the process of healing. It may be that something has come up for her that she needs time to work out on her own. She may seem angry for a while, but then process it and come back...
If, after a while, she still refuses to allow you to see her children, contact a lawyer and see what rights you have.
In the meantime, hang in there... you are their grandparent and they love you and won't forget you evn if you don't see them for a few weeks.