This is anecdotal, but every so often my sister's computer will have the same issue, with the "insert boot media" message. In her case, we need to play around with the BIOS, put her SSD in the boot order, then change the boot order - and it doesn't always take. It appears in her case to be related to really poor power. I grabbed an old UPS - its battery needs replaced, but it should at least help to clean the power up. Since doing that, her computer has only had the problem surface with a complete blackout. Your problem likely isn't related, but I just thought I'd throw it out there.