Runtime errors occur as the kernel is loading. Error messages are displayed on the screen or written to the
/var/log/messages file. Boot loader problems display messages to the screen; no log file is produced. Kernel oops are errors in a running kernel, and error messages are written to the
Excellent documentation on the Internet exists for troubleshooting just about every type of error that GRUB2 or the kernel could give during boot. The best way to find this documentation is to go to your favorite search engine and type in the keywords of the error you received. Adjust the keywords you use as you focus your search.
If you have GRUB problems, the GRUB manual is online ...