When you are an employee of a company, you are charging that company your hourly agreed upon rate when you were hired. They are not giving you that rate. You are not earning that rate. You are charging that rate. It doesn't matter if there was any actual work done on the day or not. By the Fair Labor and Standards Act(FLSA), being in your office constitutes as “Hours Worked” and is compensable. If your employer changes your hours or doesn't pay you for that time, they are stealing your wages and you should either seek a lawyer, file a complaint on the department of labor website, or both.
If you do file a complaint or get a lawyer, make sure you get all of the documentation you can for either the Department of Labor or a lawyer. They may be able to help you get all of that pay back and possibly more, called liquidated damages. Not to mention, the company will be fined to hell and back if they are found in offense by the federal government.
This is not to say “go to work, don't do work, and get paid for it.” If you take a job, you should make an effort to do it and they have every right to fire you, if you're not contracted, and not doing your job, however, you should also know your rights and what you should be compensated for so that you're not getting shafted. It is in the company's best interest to make money, and it will do everything it thinks it can get away with to make that money, including shorting your check. Don't blindly trust your employer to pay you correctly, always check behind them and hold them to account.
This also counts for work from home.
Have a great rest of the week!