I've just been loaned an IBM Thinkpad laptop. It has Linux and WinXP
installed on it. When I first turned it on several days ago, it booted
to Linux, then, without asking for a login, came up with a Linux menu
allowing me to choose between Linux and Windows. (It wasn't connected
to the internet at the time.)
Today, I tried plugging an ethernet cable into it, then turning it on.
It booted to linux and came up with a screen wanting me to log in. I
didn't think that was supposed to happen, and I certainly didn't have
a login, so I selected the "reboot" option from a menu. It shut down
properly, but failed to reboot. One of it's lights (sort of a tilted N
or Z inside a circle) remained on, and so did the battery light, but
the screen stayed black. I turned it off and turned it back on. The
Z/N light came on again, and it beeped at me about six times, but the
screen stayed black. I unplugged the ethernet cable and tried again.
Same result. Any guess what's wrong with this computer?

Hi gremlin-ga,

I'm posting this as a comment because I don't have any further
evidence to back it up, other than personal experience.

I had a similar problem with my own Thinkpad. I'm not saying it's the
same thing, but it turned out that (as far as I can remember) my
system board was faulty.

Failing that, you could check that the power cable isn't faulty. It
may appear that it's charging the battery, but a loose wire could be
the problem - I've also had this problem.

cobrien-ga

proberly the first screen u see asking u to select which Operating
system is not a linux screen at all but ratehr a LILO or another boot
loader asking u to choose which OS u want to boot.and somehow LILO
thinks that that u no longer wish to choose between two OSs and is
presenting u with the option of Booting to Linux only . u should boot
into linux and set the option.

Try a full power-off.  To do this, uplug all cables, remove the
battery pack, go have a coffee.  Come back in 5-10 minutes ...  I find
lots of otherwise intractible hardware problems disappear this way. 
Costs nothing (unless you have to pay for your coffee!), so give it a
shot...