Microsoft has released an update recently that causes ordinary files downloaded from the Internet to appear “harmful” to your computer. We hope that Microsoft releases a fix soon as it is causing problems for a large number of users.
View this article from Infoworld.com for more details.
If you have been unable to open Excel files that you have downloaded from trusted sites, follow these steps as a workaround for this issue.
Step 1 – In Excel, navigate to File > Options > Trust Center.
Step 2 – Click on the Trust Center Settings button.
Step 3 – Navigate to Protected View (in the left column).
Step 3 – Uncheck the box highlighted in the screenshot below.