If you are in a protected class, such as over 40, disabled, minority, and you were replaced by someone who is not in a protected category, you may very well have a claim. Also, if you are being retaliated against for asserting your rights under a state or federal law such as those governing disability leave, overtime or workers compensation, you may have a claim. Finally, if you were terminated because you had previously complained about discriminatory conduct that affected you, you may have a claim.
You may want to call an employment lawyer if you fit into either of these categories.
If you are a member of the union, you are protected by a collective bargaining agreement that covers these circumstances. If you are not a member of a union, the answer is no.
In Pennsylvania, you are an at-will employee. That means that an employer can terminate your employment with or without cause. Likewise, the employer can choose to realign its business as it sees fit. In other words, the courts will not interfere in this circumstance. If you remain unemployed, or simply would like to go back, you may want to contact your employer (who may not have thought to contact you before posting the position).