My pc has been having a problem where when I boot it up, it allows me to sign in to my account but then it instantly freezes and then after around a minute it shuts down. This is often followed by, upon rebooting, a black screen appears with a changing number-letter combination in the bottom corner (c5, 99 etc) and I am currently unable to use my pc at all, any thoughts on what the problem could be caused by and how to fix it?