Hi,
Since yesterday, when your phone was working properly, did you drop it, drop water on it, have it in the bathroom while you were taking a shower or bath? Did you upgrade your operating system, even if unknowingly? Go into About phone to check if the OS is different from what it was yesterday? Did you download any apps other than from the Google play store?
Think about possible incompatibilities. Cellphones don't stop working for no reason.
BTW, thank you very much for your prompt reply. Much appreciated!