If you were accused of plagiarism and if it was an accident, it depends on how it was an accident. Was it because the professor gave unclear instructions or was it because you, say, failed to properly cite the references? Was it because there was a misunderstanding between the professor's expectations and what you were supposed to do on the assignment? It really depends on how was it an accident. Many schools will not necessarily distinguish between intentional and unintentional plagiarism. That's another consideration. Some schools will have a policy that a student would not be, say, culpable for unintentional plagiarism, although that's generally the exception and not the rule. It really depends on what the accident would be that led to the allegation of plagiarism. Being accused of plagiarism at any level of higher education, be it college, a master's program, a Ph.D. program, is incredibly serious. It can have tremendous short and long-term consequences.
Having an experienced academic misconduct attorney advisor will be your best ally in helping to understand and navigate the process. They should be involved as early as possible in an academic misconduct case.