Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer
I would appeal this decision as they cannot simply stop your tax credits just because your ex has not completed his form - this has no impact on your claim.
So please advise
Since May has this always been a sole claim (just for you)
Is it just for working tax credits or also for children's tax credits
What information do the Tax Credit office state your ex needs to provide
I can then advsie further