You may have a cause of action, but it is not certain. This is an evolving area of law (the law is essentially being made right now) and there are competing rights and interests: on the one hand, it is illegal to access another's social media or emails in most situations without authorization, but on the other hand, employers have broad rights to monitor employee computer usage at work--which includes the right to monitor their emails and web browsing, if done over employer-provided machines, internet connections, etc. Further muddying the waters is that you left the Facebook account open at work, which could be likened in some ways to leaving a private letter or inappropriate literature open on your desk--if employers see something an employee has, even accidentally put into the public sphere, they can take action. That said, there  is a significant difference between the employer acting based on the open page of your Facebook account, which he/she could have seen when walking by your desk--and sitting down at your desk and paging through your account.Depending on the exact circumstances, you may have a cause of action; it is worthwhile to consult in detail with an employment law attorney.