A few times per year, the question comes up. A publisher is in breach of contract, non-responsive, disintegrating before the author's eyes. "How do I get my rights back? What do I have to do?"
First of all, you have to make sure you have a legal leg to stand on. If your contract has a "either party may cancel with X days notice" clause, obviously you have a right to cancel the contract at any time. If you are buying your way out on a release clause, again...you can do that at any time allowed by the contract. If the contract is in breach, you have the right to take whatever steps you agreed to in contract, in case of a breach.
You do NOT break a contract just to break it. You signed a legal agreement, and unless you have a legal reason to sever it, you should ride the contract out.
How you get your rights back depends on your contract. If the contract is in breach, it should have steps you should take. Your certified letters to the indie/e publisher SHOULD reference precisely what's wrong, what the breach is you're calling into play. The contract probably lays out how much time they have to remedy the situation, before rights revert to you, and still you MAY have to send a second certified letter, telling them they have defaulted and you demand immediate release of your books, but usually not. You can also reiterate that in your letter, their time frame to correct the breach, if such a thing exists in the contract.
Some contracts have a "may be canceled by either party with 90 days" sort of clause. If it does, remember that you have that 90 days to ride out, even when you invoke the clause. The publisher MAY release you earlier, but legally, you can't force them to.
And, they still owe you your royalties, either way. No matter how you leave the contract, they owe you what they owe you, though you might not see it.
If they sign for the certified letter, you have your clock for the time frame, based on that. Signing for it means the court will assume, rightly or wrongly, that they read the contents of the letter after signing for it. If they REFUSE the letter, do not open it. Save it, with all stamps and such attached to show it was refused. You may need that later. Hopefully not, but if the publisher refuses it, you may not be dealing with a reasonable human.
Unfortunately, to my knowledge, if they refuse the letter and refuse a subsequent letter, stating that they have been in breach for X amount of time, you have to formally break with them publicly. As much as I loathe that sort of approach, it may become necessary. If you, for instance, haven't been paid for your sales in months and have no contact with the publisher, warning off the readers may no longer be an option plan. You can't allow the books to continue selling, knowing you won't be paid for them. As long as you can keep it out of the public eye, try to do so. Warn other authors off privately, of course...since no one wants to see another author walk into a pit. Contact Piers and P&E about the situation, definitely. But, try to keep the readers out of it, if you can. Left no choice, go there, but don't do it lightly, and give the publisher response time, no matter how hard that seems.
The problem remains, at that point, you cannot sell the books to another publisher, while they remain for sale with the old one, even if you have taken all contractual steps to remove them, even if the contract is in breach, even if you no longer consider the contract valid. Nor can you sign them elsewhere, in many cases, unless you have a formal release, signed by the publisher, even if they are no longer selling at the publisher site and other outlets.
So, what do you do? As horrid as it sounds, you may have to actually file suit/engage a lawyer to file a cease and desist, if they won't take the books down and give you a signed release. Ideal? No. Of course not. No one wants it to come to a court case, but if it does, it does. It's the risk of contracting with ANYONE. But, try non-judicial routes first. A reasonable publisher will know that being in breach puts them in a bad legal position, and a bunch of authors banding together to file suit or hire a lawyer is an ugly moment. With all likelihood, it won't progress this far.