Can an employer cancel your weekly days off and force you to work without any additional pay or compensation?

Unless you have a written employment contract, the company can change your work schedule as it pleases.  However, unless you are properly a salaried employee, you can't be made to work extra hours without being paid for them, and that includes overtime if there are enough extra hours.

Under the "at will" employment doctrine, the employer can do pretty much as it pleases, as long as there's no illegal discrimination or other legal violation involved;  the worker's hours can be changed, working conditions can be changed, people can be fired, and the law doesn't ordinarily require the company to even have a reason.  The other side of that is, you can quit any time you want to.