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

Any person can file a representation by way of opposition on Form 7A after publication and before grant. Wrongful obtaining by the applicant, or someone the applicant derives title from, is a listed ground (section 25(1)(a); rule 55).
The Controller considers the representation only after a request for examination has been filed, first checking for a prima facie case; a patent cannot be granted until six months after publication (rule 55).
Opposition is limited to the grounds in section 25(1), so frame it on wrongful obtaining and, if true, on lack of novelty over your own earlier publication.
If you are a co-inventor, also consider a claim to be named as inventor before grant (section 28(3)).

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.