That's strange, Carys, to get a second line yesterday and nothing at all today.
Did you have much to drink before today's HPT? (I'm sure you wouldn't have as a 2 year TTC veteran, but it's worth asking)
Do you have any symptoms? Have tried doing another one? Maybe use a cup instead? Tomorrow morning with FMU might give a different result.

Can you tell that I'm quietly excited for you? Good luck!