In which situations can the prayer (salat) be invalidated?

It is forbidden (haram) to break the prayer without a valid excuse. However, in some cases, breaking the prayer becomes obligatory (wajib), in others permissible (mubah), and sometimes recommended (mustahabb). If there is a danger to a person's life—for example, if someone is attacked or calls for help after falling into fire or water—it becomes obligatory to break the prayer in order to help them. Breaking the prayer to prevent the loss or theft of property is permissible. (Ibn Abidin, Radd al-Muhtar, 1/654-655) For someone praying alone, it is recommended to break the prayer in order to join the congregation and attain the virtue of praying in jama'ah.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu