What happens depends on how far the work has gone.
Before a milestone is funded
Nothing to cancel financially. Withdraw the project or decline the proposals.
Funded but not started
Cancel the milestone and the funded amount returns to you in full. Payment-provider processing fees may not be recoverable, and where that applies it is shown before you confirm.
Started, not delivered
Either party can request cancellation. Where you both agree a split, that is applied and the remainder returns to you. Where you do not agree, it goes to dispute resolution.
Cancelling work an expert has genuinely started, without agreeing payment for it, is not reasonable and a dispute will reflect that.
After delivery
You cannot cancel a delivered milestone. Request a revision or raise a dispute.
After approval
Approved milestones are closed and non-refundable.
Cancelling the whole project
Unfunded future milestones simply do not get funded. Tell the expert rather than going quiet. They may be holding time for you.