Not a lot you can do.
I had a recent case where a client actually deposited a very large amount in escrow, we were all set to go and ... nothing.
I eventually closed the job. I can only assume some personal matter or a change of mind.
It does happen.
Annemarie, as others advised Sally above, if the milestone was funded and you used the "submit" button, you don't have to do a thing. The client has 14 days in which to request revisions. If that doesn't happen, the funds will automatically be released.
Not often--I'd say maybe somewhere between one in five and one in ten clients never approves the final milestone. Of course, if it's milestone 2 of 4 (or whatever) I have to point out that I need them to set up the next milestone before we move forward, which necessitates approving the prior milestone. But, if it happens at the end of the project or in an ongoing project that doesn't yet have a next step, I just wait it out and it gets approved in 14 days. I've heard a few freelancers say that they've received revision requests on day 12 or 13, but that's never happened to me. Clients generally either approve within the first day or two or the milestone auto-approves.