Don't worry, I'll help you find out what happened and get your computer back into normal working order! 
Your grandson is right that removing a USB stick or shutting of your computer while files are transferring. Doing so has a small chance of causing those files or the USB stick to become corrupted and unreadable!
If you can, you should also avoid turning off your computer using the power button, since this can cause unnecessary wear and tear to your computer's electrical components and reduce its operating life. If you have the option, you should shut it down with your operating system to minimize component wear and avoid the possibility of damaging your files.

From your description, it sounds like you encountered an operating system error that caused it to crash. It may look frightening, but operating systems can sometimes detect a crash and fix it without you needing to turn of your computer. You should be safe this time to shut if off, but I would recommend waiting a few minutes to see if the computer ends up fixing itself first. 

In the meantime, could you tell me a bit more about this problem? Is this the first time this has happened, or have you encountered it before? If so, is it a regular problem? 

I'll also need know what type of computer you have and it's specifications to give you the best support I can.
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