default
To fail to fulfill a promise or an obligation.
The word default can be used in several ways, but is typically used as a verb, for example, "she defaulted on her mortgage loan payments".
You can also say that someone is /in default/, which simply means that they have defaulted on an obligation, but expresses the idea as a situation rather than an action.
Finally, you can use the worddefaultas a noun to refer to the failure itself, for example, "his default led to him being sued".