I have to generally agree with Betty here.
In the strict sense of the law an employment relationship is a "contract", even "at will" employment, it is just not a contract for a definite period of time.
Although a contract/agreement was entered into as to terms of available hours, the question becomes, can it be altered, at least under your descriptive facts??
You are of course free to consult a contract attorney, but I see no remedy, I am not an attorney though.
IF such agreement is breached by the employer and you can NOT work the hours they demand, they of course will let you go. This may qualify you for unemployment benefits.
IF you can work them, even if a hardship, do so, and then try to look for other work.
Best wishes.

