Failure to start after boot loader update

Hey everyone!

Hoping to get some assistance on an issue I’ve run into.
I updated the bootloader to the latest SPL and U-Boot files (V1.0.6).
After powering off then on again, the screen receives no input through HDMI and only the red light comes on.
I’m guessing the bootloader in the SPL flash has been updated incorrectly.
The UsbFlashTool in the starter guide looks like the best option but I don’t have access to a windows system.
Any advice on using UART, being able to boot off the SD card, or any other ideas would be greatly appreciated.