A computer can crash at anytime your are using one from just running normal everyday tasks or to if your heavily using your computer. And there are different types of crashes for different reasons.
The reasons why these happen could be anything from software corruption to faulty hardware.
If your computer has crashed and you have tried to turn it off and back on again and you are still experiencing issues you maybe need the help of someone that can fix your computer.
This type of crash is most common with Windows computers. You will know you have crashed into a blue screen as the screen will turn a shade of blue and you will receive an error code with some information. The error code and information given is key to the cause of the problem.
Depending on the settings of the Windows installation, the error code will stay on the screen until the computer has restarted or it will restart after a time that the crash dump is collected.
You will know the screen has frozen when the mouse may have disappeared or wont move while your using your computer, you might hear a constant looping sound as well. Your keyboard also wont work and you wont be able to do anything which means you will have to hard reboot your computer by powering off at the mains and turning back on or holding the power button for 10 seconds to force reboot.
If the issue consistently persists it may be a problem that needs to be looked at even thou its not a good indication as it is.
Black screen depending in what situation it is experienced could be a software, hardware or peripheral issue preventing the computer from loading or the signal might have been lost to the monitor. In a way it is a crash because its not working like it should but not in the way of the blue screen and the picture frozen on the screen.
Screen showing random lines of text
This type of crash you will most likely have lines of text similar to code as well as part of the image of the operating system still showing. Usually you might see this on MAC computers or maybe Linux type computers but also possible to see it on Windows system depending on what version and how it was setup.