To see the output from when your computer started, run dmesg(8). If it has been replaced with other messages, look at /var/run/dmesg.boot.
The message buffer is used to store kernel log messages including console output as well as "normal" kernel messages; run dmesg -a to get the full message buffer contents. Typically these other log messages fill up the buffer so the kernel messages get overwritten. The logging of console output can be useful at boot time, but is often less important later since it is often just syslog messages that are already logged elsewhere. You can arrange for only boot-time console output to be recorded in the message buffer by adding the following line to the end of /etc/rc.local , creating that file if necessary (you could also use /etc/sysctl.conf, but rc.local is better because it happens a bit later):
sysctl kern.log_console_output=0
Finally, you can increase the message buffer size by recompiling your kernel with a custom MSGBUF_SIZE setting.