Depending on whether or not you had an actual contract with them, I would assume they are in no obligation to send you work as it is was on a "freelance basis"...

If you had only a verbal agreement to do work on assignment, then I doubt you would have any recorse to talk about.

In my opinion, in good ethics, they atleast owe you a decent explanation to what went wrong, or atleast comment that the assignments are finished and they nolonger have work to assign you.