Can my employer increase my hours without increasing my salary?
Labor laws differ by state. Generally a company pays you a salary when you are an exempt employee, and will pay you an hourly wage when you are a non-exempt employee. While there are federal guidelines, there may be additional rules and case law at the state level.
In California, an exempt employee is typically a professional who has significant discretion as to the manner in which they carry out their daily duties. I'm an exempt, at-will CA employee. As such, I'm basically at the mercy of the employer - we don't get overtime pay, we have a fixed annual salary. If they ordered me to work more hours, I would either have to say yes, or seek other employment (lucky it's not that kind of place).
This may also depend on things like whether there are union rules in place, whether you are an at-will employee, whether you are even an employee (vs. a contractor). It would help to have some context to give a more specific answer.