Tax refund

Hi, I worked into work today to find hmrc letter stating I paid too much tax in year 2023 - 2024 and it says that's been rolled over for a future year.

Then it says I paid too much tax in 2024- 2025. I'm essentially owed thousands. I found this later today after 3 weeks and on it says they will send the cheque straight away however they are sending these letters to my old workplace. (Same organisation)

I dont know what to do. What if someone else claims my cheque? I don't have a hmrc online gov account should i do that? Anyone got advice.

My tax codes never got changed . Can I get both refunds?

Thanks.. I'm a newbie!