[ubuntu] ubuntu 18.04(usb drive) and Windows 10 grub promt while booting.

Hello,

Mine is a windows 10 laptop.
I recently Installed ubuntu 18.04 to a usb drive. While performing the install, I had selected usb drive as the location to install boot loader.

It all goes fine, if I boot with usb plugged in. I have the option to go to windows 10 or ubuntu.
When I boot without usb drive, I am taken in to a grub prompt. Where I have to type 'exit' to continue boot in to windows 10.

How can I fix it?

Thanks.