Demotion without cause generally depends on your status and applicable laws:

• At-Will Employment: In this case, your employer can lower your salary, demote you or even fire you without cause and without incurring liability, as long as the demotion is not based on factors like retaliation and race, gender, age, etc.
• Employment Contract: Since it could include conditions under which a demotion can occur. Failure to adhere to the terms of the contract by the employer could be ground for the employee to file an appeal for unlawful demotion.

In case you believe you’ve been unlawfully demoted, the first thing you should think of doing is consulting a legal professional (employment lawyer) on the matter, since he can help you understand whether the demotion was unlawful or not. Second of all, documenting everything is the smart thing to do since it could be crucial for proving that the action taken against you is injust. Third of all, if there are reasons to believe that this demotion violated labor law or company policies, it is best to go through the internal grievance procedure or file a complaint to HR, it is also possible to file a formal complaint to government agencies like the Equal Employment Opportunity Commission (EEOC). Finally, you can always resort to settlement or mediation to resolve the issue in hand without litigation.