My employer refuses to pay any overtime if I work 45 hrs in a 40 work week I get a payroll check for 40

Under the Fair Labor Standards Act, every hour in a week over 40 must be paid at time-and-a-half your hourly rate. Under the FLSA, a "workweek" is just a recurring seven-day or 168-hour calendar period. An employer can't get out of paying overtime for one week by giving you "comp time" during another week. For example, if you get paid $10 per hour, your overtime rate is $15 per hour.  In situations where the employer knows he is violating the law, the FLSA lets you recover twice what you are owed, going back either two or three years before the date you file a lawsuit.  This can add up, especially after you double the unpaid wages.  You may be able to recover attorney’s fees as well.  The FLSA also permits “collective actions” where one employee can try to make claims on behalf of himself and other employees.  This can be helpful when one employee’s claim is small but there are a lot of victims of the same practice.  Without knowing your job duties or your employer, I can’t say whether you might fall under an exemption to the overtime provisions, but these are the general rules.