Answer: Virus attack or hard drive crash.
Explanation: In the event of this happening there are two possible things that has likely happened.
Virus Attack : There might have been a virus attack on the device.
Hard drive failure or crashing: The hard drive has failed or crashed.
Solution: You can try to reformat the system to handle any case of possible virus. Also ensure you check your hard drive too.
In 2017 the estimated cost was $245,340
In the <head> part of your HTML, you reference the CSS like this:
<link<span> rel="stylesheet" type="text/css" href="thecss.css"</span><span>>
</span>
If it is still not working, there could be many kinds of mistakes. Take it one step at a time and start simple, or share your HTML and CSS so we can have a look.
I think it is better to ask questions and troduce yourself