At Symbol Replaced with?

Sounds like you somehow switched the keyboard layout from United States to United Kingdom. With UK layout, shift + 2 key gives you quotation marks instead of the @ symbol. Check your keyboard layout configuration in Control Panel > Region & Language > Keyboards & Languages (tab).

Why can't I get the at symbol on my keyboard?

First is to make sure that the keyboard language is set to United States. Go to Control Panel then click Region and Language. Once open, click on Keyboards and Languages then click on Change keyboards and make sure it's set to United States. If it doesn't work, uninstall/reinstall the Keyboard driver.

Why do I get inverted commas instead of the at symbol?

This happens because your keyboard layout is changed to UK from US where @ symbol appears where " symbol is there. Step 1. Windows Key+I to invoke Settings > Time & Language > Region & Language > Add English (US) or any other appropriate language.