Sudden change of phone display without notice resulting in malfunction

My wife and I have the same LG G2 phone bought in 2014 with AT&T. Last week, her phone screen was suddenly changed with different color, theme and display format of Apps. Today the same thing happened to my phone. As the results, some of the Apps do not work anymore. For example, there is a "Weather display" on the main page by AccuWeather.com, which came with the original phone. It does not work now and cannot be downloaded either.

Question: is this change by the company without notifying its customers? or is this viral infection?

How to deal with this change? Thank you. Rich