If the accident is out and you have not had your brush cleaned after use, then it is not necessary to throw it out. It can in all probability be saved.
Grease the brush in Sterling Brown Soap and wrap it in an airtight plastic bag. Leave it for 3-4 days and then rinse thoroughly with water.
The same procedure can be used if a brush has been used for oil painting. However, the brush only needs to lie for 1-2 days before it can be cleaned off.