I left my T400 on overnight running on battery and it completely discharged and now it won't start up. The plug and battery lights do illuminate when I plug it into an outlet; when I turn it on, the num lock and caps lock lights illuminate briefly, and the power status indicator light illuminates. It makes a few whirring noises, and then just sits there.
This symptom could occur on some earlier T400 systems with downlevel BIOS if the battery were completely discharged. There are several methods to reset the system, and then once reset, the latest BIOS should be installed.
1) Remove battery and AC adapter and then press and hold the power button for several times for about 3 secs each time.
2) Re-attach battery and AC and restart. This should resolve in most cases
If the system has not reset, you may need to remove the CMOS battery and let the system sit for 30 mins to an hour and repeat steps above, then reconnect CMOS battery. Additional information on the following steps to remove the palmrest can be found in the hardware maintenance manuals on the Lenovo support site http://support.lenovo.com and videos for how to remove palmrest and keyboard are available here(T410 model shown by mechanically similar)
1) Remove battery and AC adapter
2) Remove the 5 screws on the bottom of the system that have the icons for keyboard next to them.
3) Carefully lift off the palmrest and place it toward the front of the machine so you have access to the memory sockets. If needed, carefully disconnect the ribbon cable connector that goes the the system board - it lifts straight up.
4) Lift up the lower edge of the keyboard if needed to access the round yellow CMOS battery which looks like this...
The battery attaches to the system board with a 2 wire connection - red and black wires. Locate the 2 pin connector and carefully unplug it. Leave it disconnected for 30 mins to an hour, then reassemble and repeat the 2 setps in the first proceedure.
5) The system should now restart and will give a time and date error. Enter BIOS to set time and date, then save and exit via F10.
The system should now be running properly. Download and appy the latest versions of BIOS for the T400 from the support site.