What should we do regarding a mouse falling into a well and using the well water for a period unknowingly, then discovering the matter recently, according to the teachings of Islam?

If a mouse falls into a well and dies, it must be removed. If its presence changes the water (its color, taste, or smell), then the water is impure (najis); otherwise, it remains pure (tahir). The ruling does not differ based on how long it stayed, but a prolonged stay may lead to a change. This is supported by the hadith of Bi'r Buda'ah: "Water is pure; nothing makes it impure," and by the sayings of Ibn al-Qayyim, Ibn 'Uthaymeen, and Az-Zuhri. To purify the well if its water has changed due to impurity, the change must be removed by adding a large amount of water, by drawing out the water, or by adding substances that eliminate the change.