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The general rule in VA is "at will" absent an express agreement to the contrary. At will employment can be terminated or modified for any reason not amounting to discrimination on the basis of a legally protected trait (race, religion, gender, etc.) or retaliation for engaging in certain forms of legally protected conduct (filing a wage claim, taking FMLA leave, etc.). It doesn't matter whether the basis for termination/modification is fair, reasonable or even TRUE.
This means you can be disciplined or even terminated based on the false belief that you failed to timely report an injury. It is certainly unfair if that happens, but there is no law that requires your employer to be fair or to keep employing you.
So, I'm afraid that all you can really do in this situation is attempt to reason with your employer. This means submitting a written explanation for what happened, being sure to mention that you have a witness who will corroborate that you reported the incident to your supervisor. You don't want the tone of your written explanation to be hostile or aggressive. Your only power here is the power of persuasion, and persuasion is best accomplished through an open, calm and even-handed approach to the situation.
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