Open control panel.(win key + r. Type control and hit enter)
While in icons view, click on power options.
Click on Choose what the power buttons do on the Left hand side.
Uncheck turn on fast startup. (you may need to press change settings that are currently unavailable before this)
Click on save changes.
Source: https://askubuntu.com/questions/806570/cannot-access-windows-partition-from-ubuntu