Rob Morley wrote:

I guess something with a melting point somewhere between 60 & 100
Celsius is lodged in the trap. IME cooking fats, which are a common
cause of blocked drains, melt at lower temperatures, but it might be
something like candle wax.

Try adding some detergent to the water - this should emulsify whatever it is
and stop it setting again as it cools down.

Dave