Can i use the opt which was not used after my masters.

I don't think so unless its been less than 90 days. The OPT must be used, i.e. you must have secured employment within 90 days from the time you graduate in a position commensurate with your degree, i.e. you can't work as a bagger in a supermarket with a MA. But also check with the DSO of the MA school in Pittsburg and see what their take is.

Nope. The moment you re-enrolled back in school for your PhD program, you gave up your employment authorization based on your MA opt. You may qualify for CPT under your PhD program but not if you drop out.

You should get an OPT period for each degree level. I'm not sure if you could go back and claim the Master's time, but I have helped people utilize unused time in the past. Much depends on the facts of the case and your facts are unique to my previous cases.