I checked the website from ASUS and came out on this page which, I believe, states the problem you are having. Take a look and see if this can help you fix your system.
ASUSTeK Computer Inc. -Support-
And something else I found. It's not the same netbook you have, but from the same family. It explains how you can boot from an external device, and also how you can update your BIOS, if it's needed.
ASUS X200 usb boot enable step by step - YouTube
If you can't access the BIOS, it means it's been corrupted and it has to be installed again.
I don't know ASUS systems well enough to give you the correct information how to do it, but the ASUS helpdesk should be able to help you with it. Worst case, you have to send it to them, so they can do it.
But I'm sure ASUS must have an easy recovery system in place to flash a BIOS.
According to the EULA for Windows 10, your contract regarding this personal disaster is with Microsoft:
You don't want help - you want action to restore what is yours from an upgrade which was pushed onto your machine and was supposed to be safe, but was not. You don't stand a chance of any damages, by normal legal means but you can push the boundaries using publicity.Depending on how you obtained the Windows software, this is a license agreement between (i) you and the device manufacturer or software installer that distributes the software with your device; or (ii) you and Microsoft Corporation (or, based on where you live or if a business where your principal place of business is located, one of its affiliates) if you acquired the software from a retailer. Microsoft is the device manufacturer for devices produced by Microsoft or one of its affiliates, and Microsoft is the retailer if you acquired the software directly from Microsoft.
Write to them - not support - customer relations. If you were in the UK (I now realize from your time zone you are not, and you give no indication of your location or nationality) I'd do this:
Head office is Reading.
Fax is better than email or snail-mail because you have a record that they received it. Give them deadlines for responses.
Offer to make a fuss* if they don't escalate your problem satisfactorily by, for instance, sending a technician round to fix the damage your failed upgrade has done. Don't let the machine out of your sight.
Tell them that you will take them to small claims court to recover the damages for the loss of your machine.
* BBC and other press, local & national, MP, local trading standards for a start. Copy anything and everything to MS, Staples, ASUSTeK, and whoever reports your case.
There's no such thing as bad publicity, and nobody wants to look like the bad guy. You'll get a resolution, if you do it right.
If you are not the only user that his has happened to, I would not be be surprised. One thing MS may fear is a class action against them.