Operating system not fond massage on my Lenovo 300?

It means that the boot files were not found.

1. This could occur if your computer got infected with malware.

2. Ensure that you don't have any external storage devices connected to the computer; example flash drive.

3. System32 a folder in Windows got corrupted, meaning that it something was renamed, moved or deleted.

These are just some basic answers.

The best option is to re-install your O.S.

Your hard drive may have failed or it may have become disconnected, either power or data.