As a first step, you might consider reviewing your contract carefully, if you have not done so already.
For instance, is there a clause in your contract which addresses this situation? For example, look for a heading entitled ''breach'' or something similar, and you will likely find provisions which discuss procedures to be followed in the event of a breach, opportunities and time frames for remedying breaches (depending on the category of breach) and communication processes to be followed. Also you might like to review the provisions in relation to termination to check whether their letter complies with the procedures set out in the provisions.
Hope this helps as a first step. Let us know how it goes, and we'll see what other contributors have to say as well.