no Boot from HDD after Restart

[Fohnbit]

Hello

I have a Intel NUC PC and try to install Mint.
I copy the ISO on a USB stick and start from the USB.
I choose that the whole HDD should Mint use.

After complete the installation the system reboot and start again from USB ... when I put the USB out, then he found no boot device.

How can I fix the boot sector?
Before it was windows 7 on the HDD.

Thank you!