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Maternity leave is job protected leave and your employer was required to keep your position for you or provide you with a comparable position.
This did not happen and given that someone else has your job the employer has simply ignored this legal obligation.
The employer has therefore acted unlawfully and contrary to the Human Rights Code.
You can file a complaint with the Human Rights Commission or the Employment Standards Branch. I suggest you call both of them as soon as possible.
You will be entitled to damages but I can not in the position to be able to assess what they will be but they will be higher than regular termination pay.