Refund for Cancelled HK Express Flight

I hope someone familiar with the airline industry will be able to offer advice.

Last year, I took an evening flight on HK Express that was cancelled after we sat on the tarmac for several hours praying the mechanical problem would be resolved.

After we were told to get off the plane, the HK Express staff informed us there would not be another plane available until later the next day which meant a lost workday. It was now very late at night.

I was given the option by HK Express ground crew to purchase a ticket for an early morning flight on another airline and request a refund.

When I contacted their customer service team, they claim the flight was never cancelled, it was only delayed a full day.

This is very different from what the HK Express ground crew told me. Anyone been through this and can offer advice on how to proceed with my refund request?