Breaches could be material and non-material. Although it’s possible to send a Breach of Contract Letter in both cases, people usually use this document for material breaches. Non-material breaches are challenging to prove, and they usually won’t result in terminating the contract.
If the other party doesn’t respond, you should send them another letter, referring to your first letter. If they ignore that letter as well, you may need to consult your lawyer about what your future steps you should take. Don’t worry, there are a lot of things you can do, you just have to choose the one that will work in your particular case.
If you’re writing a Breach of Contract Letter yourself, without a legal advisor, make sure it sounds formal and objective. Avoid being emotional and using phrases that sound like you’re accusing the other party.