There are lots of solutions because because actual faults cascade problems.
1) Fix water leak through pollen filter by sealing the cover/windscreen using correct adhesive strip and replacing clips properly.
2) Check and repair/replace blower motor. The bearing become stiff due to water getting to them.
3) Check/replace resistor pack, or just the thermal fuse. If the blower became stiff it doesn't produce enough airflow to cool the pack and the thermal fuse protects it.
4) If 2 & 3 were OK, check the switch. Usually this is only a problem if there is no blower at all.
Martin