First of all it is my friend's laptop which I am talking about. He owns an around five years old Toshiba laptop, which he upgraded from win 7 to win 10 three weeks ago. Since then the laptop occasionally starts up with "no bootable device found"-message. He resets it and it starts up just fine. If it is necessary I think I can give the more precise type of the laptop too. Sometimes (after booting and running fine) it pops up the message to check the ethernet cable connection, and finds no signal. After resetting, this problem usually disappears too. What could cause the problems?
Sorry if I said something wrong, I'm not a native english speaker.