Someone filed my invention in their name. What can I do?

The route depends on how far their filing has got. Both the timing and your evidence matter, so start by finding out the status.

Urgent - a deadline or loss of rights may be close

A representation by way of opposition can be made only after publication under section 11A and before grant (section 25(1)). Watch for publication.
If you are a co-inventor of what is claimed, a claim to be named as inventor can be made before grant (section 28(3)); it cannot be made after grant (section 28(4)).
Consider filing your own application for your invention. Ask an advocate or patent attorney how section 29(2) and (3) would treat the other filing: a prior publication taken from you without consent may be disregarded only if you act promptly and the invention was not already commercially worked beyond reasonable trial.
Preserve evidence of the original disclosure and how the other party obtained it.

The honest answer usually needs someone to look at your actual disclosure, your timeline and the prior art. That is a conversation, not an article.