Boot device not found. Please install an operating system.

[PattyOhio]

Boot device not found. Please install an operating system on your hard drive. HP Pavillion all-in-one. Possibly infected with malware after downloading a user manual for a Dremel tool. Computer went haywire after clicking on the manual and not will not boot up.

[BeemerBiker]

You will have to boot a recovery USB or CD and select the repair option.

If your PC is a 2016 model or newer, you can get it here (do not do the install, just the repair)
https://support.hp.com/us-en/document/c06162205

Alternately Microsoft has and ISO image of win10 that can be used to do a repair.  There is a link to where to download but no need to to the custom install, just a repair

https://answers.microsoft.com/en-us/windows/forum/all/how-to-perform-a-custom-installation-of-window...

Once the repair starts up if it finds an operating system then select a recover point that is dated before you downloaded the malware.

If no OS then you will have to actually do a boot  repair which can be problematic.  Post back here if a problem.

There is usually no problem downloading manuals.  The problem arises if the web site wants you to use their downloader.  When you hover over the download button, look in the status at the bottom of the browser to see if the link is to a manual or to an executable program that does the download.  If the file downloading ends in .exe or .msi then cancel the download.