It's a common problem for Microsoft Excel user. I suggest to use " / " instead of ALT key for quick solution. But it is not a permanent solution. In this article I will explain how to solve this problem.
Solution:
Step 1:
Go to: C:\Users\User_Name\AppData\Roaming\Microsoft\Excel (replace user_name with your user name). You can directly reach here by copy and pasting the following in Explorer address bar: %appdata%\Microsoft\Excel and see if is contains any files or folders.
Step 2:
If you find any file/s folder/s then move them to another location and make this folder blank.
Step 3:
Open the Excel program and try the ALT shortcut key now.