former employer giving bad referances

I worked for a company for four years. When I went on maternity leave I came back to a new boss. Let me add I was promoted to middle management and never got wrote up.My new boss made coments I found uncomfortable so I brought it to human resources attention.
Which took action but my boss made it awful. He insisted I work split shifts example 8:00 am until 12:00 pm go mome and come back at 4:00 pm until 9:00 pm everyday except sunday.
I found out I was not salaried but hourly after I had worked over two years over time. I resigned in disgust but I have been unable to get back to work. I have go on interviews but it seems that whenever they do a back ground they no longer want to talk to me. I never got wrote up for anything, I gave a adequate notice. My boss was the one to fill out the forms determining my eligablity for rehire. Can I take action against the company if they are recommending I not be hired. By the way human resource was aware I was owed back pay?
I hope you can offer me some advice

You bring up a number of questions. First, it is possible that you have a claim for retaliation because of the way your supervisor treated you after you complained about him. I do not know if being put on split shifts would be enough to support a claim, but you should ask an attorney. In addition, you may have a claim for overtime compensation. However, I would hurry on that claim because the statute of limitations bars claims for overtime more than two (or, in some cases, three) years old. Finally, if your former employer is saying untrue things about you to prospective employers, you may have a claim. Call an attorney. An initial consultation is usually free.