This is most likely a corrupted/damaged BIOS. I have had this happen to one of the machines at the office where I work, processed a BIOS update that botched and would only flash white, no pattern.
I had to make a BIOS recovery flash drive. You can find the downloader for the BIOS update for that at HP's support site for your model (The Elitebook 840 G2 drivers and downloads site is here). If you run the installer on a different Windows machine, it should give you the option to create a recovery drive for another device. I often get an error with these where it cannot save to the required folder because it doesn't exist or something to that effect. You just need to create that folder in the root directory on the flash drive.
Once the recovery drive is created, plug it into the device that needs recovery and then power it on. Theoretically it should recognize the BIOS update file and process the update.