But fan still runs, and it sounds like the harddrive is spinning?
Do you have a VGA monitor available?
You can use a CRT type, (Looks like a small TV), or a flat LCD screen monitor if it has a VGA cable.http://en.wikipedia.org/wiki/VGA_connector
1) Laptop OFF, monitor OFF, connect the VGA monitor cable to the left side of the ThinkPad T42. (DisplayPort)
2) Turn the monitor on. If a CRT type allow it to warm up.
Turn the laptop on.
By factory default, the display should show on the external VGA monitor.
There are at least three display options available;
1) Internal monitor of laptop ONLY
2) Internal monitor of laptop AND external VGA monitor
3) External VGA monitor ONLY.
Press, and hold down on the Fn key, and at the same time tap once on the F7 key. The F7 key is the Display Toggle-Over key.
3) Good display on external monitor?
Problem is the video cable, or inside the Display Assembly.
Since the display goes dark after a couple of minutes, I can tell you the problem is the screen Inverter, or Backlight.
(Inside the Display Assembly)
90 PERCENT OF THE TIME the fault is a bad screen Inverter.
An LCD screen cannot produce light. It needs an additional light source.
A Backlight is the additional light source.
A Backlight can be a CCFL, or a series of LED's.
(Light Emitting Diode)
The Lenovo ThinkPad T42 series of Notebook PC's, uses a single CCFL for a Backlight,http://www.screentekinc.com/IBM__ThinkPad__T42-series-laptops-lcd-screens.shtml
A CCFL is a Cold Cathode Fluorescent Lamp. (Bulb)
Similar to Fluorescent lighting used in homes, and businesses, but on a MUCH smaller scale.
[Average thickness is 2mm. That is a little larger than 1/16th of an Inch.
Average length is almost as long, as the LCD screen is in height ]
A screen Inverter converts the power from the laptop, for the LCD screen, and Backlight.
(Older versions of backlighting that used a series of LED's, used a screen Inverter. Newer versions do not. For these replace the LCD screen)
A screen Inverter and an LCD screen are a MATCHED unit. Make sure you get the correct screen Inverter, or LCD screen, when buying.
One example of a screen Inverter for the T42's,http://battery-adapter.com/product_info.php/products_id/3381/vThinkpad+T42
Pictorial guide for replacing a screen Inverter, in a ThinkPad T42 series of Notebook PC,http://www.insidemylaptop.com/replace-screen-inverter-in-ibm-thinkpad-t40-t40p-t41-t41p-t42-t42p-laptops/
Lenovo Support link to the Hardware Maintenance Manual, for the ThinkPad T42 series of Notebook PC's,
(HMM. It is a Service Manual),http://support.lenovo.com/en_US/guides-and-manuals/default.page?
Click on the bold black subheading -
Hardware maintenance manuals
Then click on the blue -> [+] Click for files
It is a PDF file.
It may take up to 30 seconds before the first page comes up.
For additional questions please post in a Comment.