If you find that the leaves have gone dry and crispy on your bonsai the most likely cause is dehydration. The soil has dried out too much at some point and the bonsai has suffered as a result. There are a number of stages of dehydration which you can check for. If the leaves have started wilting but are not crispy it is desperately in need of a drink and you should give it water straight away. This should sort it out and get it looking it’s best again.
If it has not had water for a long time you will find that the leaves are going crispy on certain branches but not all.
Finally the last stage is when all the leaves dry up and go crispy on ALL the branches. Even at this stage your bonsai might be saveable by soaking it in water and pulling off all the leaves to further save moisture.
There are of course other reasons for leaves drying out and going crispy. These could be the lack of a good root system, spending too much time out of it’s pot during summer or diseases. With an indoor tree it could mean that it is placed too close to a radiator.
Whatever the cause the fact is still the same that it has not had enough water.