Proving a breach of contract can be complex, as the following needs to be provided as evidence:

• the presence of a contract, including an initial offer and outlining the terms, conditions, and responsibilities to which each party must adhere.  
• the other party’s negligence in performing their responsibilities.
• you upheld your end of the bargain.

the damages you suffered as a result of the other party’s negligence and failure to hold up their end of the bargain, whether financially, reputationally, or health-wise.