When working in SQL Server Management Studio, you might inadvertently hit the wrong keys on the keyboard, which will cause your mouse cursor to change from a line to a wide block.

When working in the SQL Server Management Studio Query Editor, you might inadvertently hit the wrong keys on the keyboard, which will cause the mouse cursor to go from a thin line to a wide block.

Inside of SQL Server Management Studio Query Editor, press the following Keyboard Keys
FN + Ins (without the +)

Pressing the above two keys will change your mouse cursor to a wide block cursor.

Inside of SQL Server Management Studio Query Editor, press the following Keyboard Keys
FN + Ins (without the +)

This will restore the cursor to its default state.
Pressing the keys will toggle the cursor between thin and wide.

If the above does not resolve the issue, try closing SQL Server Management Studio, then reopening it. This usually resets everything back to normal that was not working correctly.