How can you fix a boot loader error

The status led is off, neither button press can change the status led. D7 is always on blue. I read that is is most likely a failure of the boot loader. How can this be fixed

If you have a Dim D7 and the buttons don’t do anything you most likely have a missing bootloader. The only way to fix that is with a SWD/JTAG programmer. The options are listed here.

I presume you have a Gen 2 device (Photon or Electron) so you can use the Particle debugger, or a cheap ST-LINK/v2 clone. The latter will be much easier and cheaper to find.