In many cases, unless there is a contract or bargaining agreement, employees are considered covered under employment at will, which means your employer doesn't need a reason to fire you. However an employee can be wrongfully terminated if discrimination is involved in the termination, if public policy is violated, or if company policy states guidelines for termination.
Other reasons that could be construed as wrongful termination include retaliation i.e. being fired for being a whistle blower or complaining, or for not being willing to commit an illegal act when asked to by an employer.
Discrimatory reasons that can be considered wrongful termination include firing an employee because of race, nationality, religion, gender, or age.
If you believe your termination was wrongful or you have not been treated according to the law or company policy, you can get help. The US Department of Labor, for example, has information on each law that regulates employment and advice on where and how to file a claim.
Your state labor department may also be able to assist, depending on state law and the circumstances.
In addition, local bar associations often have a referral service and may even have a hot line you can call to find an employment lawyer. Keep in mind that you will need to pay for an attorney's services.
Termination and Unemployment
When you are terminated you may not be eligible for unemployment compensation. If you are not sure whether you're eligible for unemployment, check with your state unemployment office to determine your eligibility for unemployment compensation. If your claim is denied you will be able to appeal and explain the circumstances of your termination.