On some older Thinkpads I've found that a defective (stuck) key can cause a non-boot, stuck at splash screen condition. If you have a docking station/port replicator you can disconnect the laptop's keyboard, mount the laptop to the dock and use the power switch there to boot the system. If it works, then you could assume that one or more of the keys are bad and try another keyboard. If you still can't boot the system, then the problem is probably located on the motherboard.
ref:
viewtopic.php?p=506937#p506937
Note that I've just verified (last night) that the table for the T23 can mostly be used on the X200s. There are several keys on the X200s that threw codes that do not exist on T23. I'm assuming (there's that word again) that the T400 will use similar codes but at the moment I can't claim totally compatibility. I do have one T410 and several T420 systems and can document stuck key error conditions within a couple of days.
edit: Found at least one key on my X200s that causes a 'freeze' at the splash screen if it is stuck while booting the system: Pause (kind of makes sense, huh?)