Either your USB connection dropped or you don't have enough storage space on your Windows PC, and the flashing was stopped.
You can safely try flashing again, I suggest following these steps:
- Make sure have at least 32 GB free storage space on your Windows PC, and disable Brave Shields too for the web installer webpage.
- Then return the phone to the bootloader interface using the hardware buttons on the device.
- Then you press flash release again in the web installer.
If the flashing fails again, it could be an issue with your USB cable or the USB port on your PC.
You can safely try flashing multiple times with different USB ports, cables and devices, until it works.