I've had several PowerBook 140s do this to me. If it's doing what I think it is, the display turns on, but it's solid black. You can tell the backlight is on, but no picture shows up - just gray nothingness, regardless of however much you fiddle with the contrast and brightness sliders.
The fix for me is to first do a head-swap with a known-good unit to make sure that the computer is fine and it's the display that's at fault. Then, it's major surgery time.
The display needs to come apart. Once you get the top bezel off, look at the cables and make sure they're not ripped or otherwise damaged. If you're not afraid of taking the LCD out of the back of the display, do that next.
*Be VERY careful with the LCD mount screws - they're about 15 years old, and plastic tends to become brittle with age. Too much force WILL break the screw mounts, and then you'll be out of luck until you get some extreme epoxy action going on in an attempt to remount the little brass screw dealies onto the plastic.*
After the LCD comes out, remove the flappy external metallic shield dealie (may be silver and/or blue) that covers it and set that aside. Then, check the back of the LCD, especially the connectors. Remove and reseat the ribbon cables; sometimes the connectors get corroded after so many years worth of use, and a removal and reinsertion of any cables often clears it up a bit.
Usually after this step I rig it all up to see if my problem has been rectified. If the display works now, I put it all back together. If not, I proceed further into the LCD.
You might not want to continue if you haven't done this before. I doubt you'll be able to find a resource for disassembling one of these particular displays on the internet, so you're pretty much on your own outside of my instructions here.
Anyway, if you decide to press on, you're going to be taking the metal backing off of the LCD. This involves a pair of small pliers (I usually use needlenose) and a small screwdriver set. Around the perimeter of the metal backing are little tabs that are bent over the plastic. You're going to need to bend these tabs so that you can pull the metal backing up off of the display assembly. Just grab them with the pliers and twist them slightly until they're out of the way. There may be a few screws securing the backing somewhere (I can't say how many without knowing the exact model display you have). Find them and remove them.
After you get the metal backing off, you should be able to see the component side of the majority of the circuit boards in there. On these circuit boards, you're going to inspect the individual components for the following: obvious failures (burnt/broken devices), cracked or overly oxidized solder joints, leaking capacitors (there are some electrolytics in there), and anything that appears broken (cracked circuit boards, ripped cables, connectors, etc).
On my machines, usually bad solder joints or dead individual components (usually a resistor or capacitor) are the cause of the problems. Replacing or touching those up gets me a nice, crisp display (well, as crisp as an STN can get, anyway) when I put everything back together. Reassembly, by the way, is just the reverse of the take-apart process. Put the screws back in, bend the tabs back to the way they were, and reinstall the LCD into the display housing.
If the above doesn't fix your problem, then there may be something that you can't get to that's bad. Since you've got a relatively simple STN display, there isn't too much that can go bad save for those resistors and capacitors mentioned earlier, so this really should do the trick.
