Importing MP3 gives flat line

Look at Edit > Preferences… then “Directories”. This shows the temporary folder Audacity records or imports to before you save an Audacity project. Make sure you have permission to write to that folder (right-click over it in File Explorer, then choose “Properties”).

Also make sure some security app is not deleting your temporary folder contents to “help” you. If that is the problem, change the Audacity temporary directory to one that does not have “temp” in its name (typing in the name directly is the best way). If you have Norton, you can turn off its Windows temporary file cleaner.