Flashing underscore

I was working on my computer and adding a new cooling system and was re applying the paste on my processor. I put it all back together and turned it on. It apparently reset my whole computer, because it showed a message saying BOOTMGR MISSING. I got a usb with Microsoft Windows 10 boot on it...

1. What is your budget? £2000 approx. 2. What is the size of the notebook that you are considering? 15.6", 17.3" at a push. 3. What screen resolution do you want? 1080p (pref GSYNC 120Hz) 4. Do you need a portable or desktop replacement laptop? Desktop replacement though portable would be...