Losing my TA-ship, need advice I am a third year student in a U.S. university and for the past two years I was funded by dept. TA-ship.

The new semester is coming but I have not got the TA-ship. I contacted my advisor for help and he sent a note to a professor who is taking charge of all TA stuff and he also told me to hope for the best(My advisor and dept. secretary told me there would be a second round assignment). 

Here is my question: How many chances do I have to get a TA offer? And I also want to ask my dept. why this thing is going to happen (because my GPA is over 3.5 and meet all requirements). I feel very frustrated and eager to know why. Would you please give me any advise?
Most graduate programs try to be more transparent and forthright about TA funding than seems to be the case for you. That is, at the outset there would be promises made about number of years of funding, conditional on reasonably good performance in coursework and teaching. Departments are very well aware that grad students will have difficulty surviving (much less paying tuition) without financial support, so do not usually cut support without considerable prior warning, explanation, explanation of terms for reinstatement, or explanation of economic conditions and prognosis for future funding. To do otherwise is irresponsible.

I do have to wonder whether you were paying attention to disclaimers about fragility of funding, though, too. For that matter, do you have a copy of the TA offer letter? It really should spell out what you can expect in terms of funding. For that matter, no one's advisor should be "the last to hear" that their student(s) didn't get TA funding renewed.

Of course, it might be that the central administration at your university "pulled the rug out from under" your department, and the budget changed abruptly and somewhat catastrophically. But, still, I'd think you would hear something about this, an explanation, rather than just a passive non-support situation arising.