I am working in NY state as a software consultant on contract bassis ( Hourly rate) , I get a wonderful new job,
Situation forced to leave the current job with only one day notice.
I left the old job and joined new job with one day notice. Is it mandatory to give two week notice?
Ex employer is NOT paying me two week wages, because I left without two week notice.

Two weeks is a courtesy in most cases. It allows you to be rehired by that company if you need to go back because you left giving the company enough time to replace you.

Unless you had another contract enforcing a period before quitting, you should be fine. Just don't expect a good reference from them.

The ex-employer must pay you for any time that you worked, no matter how little notice you gave.

Should the employer refuse to pay what is owed, file a wage claim with the State Department of Labor ( http://www.labor.state.ny.us/workerprotection/laborstandards/workprot/lshmpg.shtm).