what is the make and model of your PC?
Also make sure that region and language is set to English.
Go to start - settings - time and language - region and language.
What is the your default keyboard?
see if it works on the online keyboard.
Try this, go to search and type osk.exe and hit enter.
this will open a virtual keyboard, see if the alt key works there.
if it does then it is hardware issue on your keyboard.