Most logs are in /var/log
. The ones most likely of interest to you are:
boot.msg - the most recent bootup
boot.omsg - the next most recent bootup
messages - a chronological collection of system messages
The latter two might contain some info on the graphics bootup process.
 You might find some archived messages with names like messages-20061105.gz
. These contain messages that predate the date-time stamp in the filename. These files may be useful, depending on how much time has passed since your problem occurred.