This does sound quite serious, but could be all sorts of things.
It may be the computer is getting too hot and certain devices are switching off due to the heat. Make sure the PC has plenty of room for the air to circulate, also make sure the fans are working and air vents are not clogged with dust.
You may need to take the cover off to check this. If there is lots of dust blow it out with a hair drier (carefully).
A message "no operating system found" means it cannot find "windows" on the hard drive, which means the hard drive may be faulty, or have intermittent errors.
Or it may be that there is a loose cable somewhere inside the case.
If you have just bought the PC, or added an extra hardware componnent, or moved it from one place to another, it MAY have a loose cable.
Again, you will need to take the cover off and check all the cables are securely in place.
Unplug it from the mains before taking cover off, and make sure you "earth" yourself on a radiator or other metal before touching inside the PC.