Blue Screen of Death (BSOD) "is a colloquialism used for the error screen displayed by some operating system most notably Microsoft Windows, after encountering a critical system error that can cause the system to shut down to prevent irreversible damage to the system's integrity."  When this happens you could get lucky and it can be fixed, or unlucky and the computer needs to rebuild from scratch.  Be prepared to deal with unexpected errors like this by backing up your computer regulary.  Article Importance of Backups talks about three strategies you can deploy to minimize any data loss shhould you encounter the BSOD