How do I

[Trirenamarylee2]

Hello my computer went completely blank and stated Boot device not found (3F0) what do I need to do ? 

[itsmyname]

@Trirenamarylee2 -- what do I need to do ? 

First, a small amount of swearing and cursing directed at the computer.

Second, your computer has a "hardware" problem. The disk-drive completely "failed".

When you try to power-on and stasrt-up your computer, it reports that it cannot read your copy of Windows from the disk-drive. 

The word "boot" comes from the phrase "pulling oneself up by your bootstraps". 

So, not having a working disk-drive means that your computer cannot "boot".

Maybe, it is something simple, namely that the "power" cable from the computer's power-supply to the disk-drive is loose, or damaged, such that no electrical power reaches the disk-drive.  Or, the "data" cable between the motherboard and the disk-drive similarly is loose or damaged, preventing all transmission of "data" from the disk-drive to the motherboard. 

If it not either of the above, you need to seek help, because the disk-drive needs to be replaced (compare to swapping-out a flat automobile tire for the spare tire),  and Windows needs to be installed onto the new disk-drive. If that task exceeds your technical computer skills, either find a "computerphile" friend to take those steps, or take the computer to a computer store, and pay for the work.

Note that if your HP computer is less than 12 months ago, you can contact HP Support, and open a claim against the HP Warranty. HP will repair/replace your computer, at HP's expense. You probably will have to ship the computer to the nearest authorized HP Service Centre, and be without your computer for about a week or two.  Maybe, the local computer-store will be faster, though obviously not at a "zero" cost to you.

[TheOldMan]

While the explanation given by @itsmyname is the most likely explanation, before taking the PC to a technician try running the Diagnostics routine,

To run the Diagnostics routine, Start tapping the Esc key BEFORE starting the PC and then press the start button.
Keep tapping the Esc key until a popup menu shows. Then pick Diagnostics from that popup menu, F2.
Report back with the results.