Hello and welcome to JustAnswer.
I'm sorry to hear about your situation and hope I can help.
Unfortunately, an employer can change an employee's status from exempt to non-exempt, as long as the employee is correctly classified as a non-exempt employee.
However, the employer cannot refuse to pay the employee for hours worked, if the employee was unaware that the employee needed to clock in and out as a non-exempt employee.
If the employer refuses to pay you the hours that you've worked, then you can file a wage claim against your employer with the Department of Industrial Relations.
You can do so using the forms available online here:http://www.dir.ca.gov/dlse/howtofilewageclaim.htm
Additionally, if your job duties weren't changed, it's likely that you've been misclassified as an exempt employee, which would mean that you would be due overtime for all hours that you've worked in excess of 8 during a day or 40 during a week, and your employer would also be liable to pay for any lunch breaks that you've been required to work through in the past.
I wish I had better news to give you, but I hope you appreciate a direct and honest answer to your question.