The washing machine isn't heating the water — what's the reason?

This is almost certainly caused by the heating element, which may have failed (due to a defect or breakdown), resulting in the water not being heated.

However, there are cases where the heating element is fully functional but still not doing its job.

Why? Because in this case the fault could lie with the electronic control board, which is preventing current from reaching the heating element due to a damaged component (for example, a relay) or a burnt wire.

In the first case, you need an engineer to carry out the repair. In the case of a burnt wire, if you have a bit of DIY confidence (and bearing in mind it is a modest expense), you could potentially do it yourself.