I changed the bulb on my Zenith Z44SZ80 and the TV picture is really dark and not bright like it used to be. The old bulb was 100w and the new one is 120w. I tried adjusting the brightness and contrast but it didn't work.

This is caused by a bulb that is dying. Adjusting the brightness and contrast on the TV is a temporary fix, but you should have a new bulb installed soon. If you need help finding the correct bulb or where to get one/how to install one, reply to this post. Hope this helps =]
I'm also glad to say bulbs have come down in price and it IS worth replacing for a high quality HD picture once again =]

It could be that the picture tubes have gone down on emission, there is a prcedure that can be tried to bring up the overall brightness but it is just possible that you will loose some picture quality. If it's a 1994 projo tv i am thinking of the three crt system, a gray scale alignment would probably correct the dark condition in your picture, again depending on how far gone is the emission in your picture tubes, if your projo tv is the dlp system, the bad news is that you could be needing some light engine repair. The color adjustment is possible with the availability of a service manual or in the case of the dlp system, light engine repairs or replacement are available from some on line stores.

You could try to INITIALIZE the tv by doing the following:
1) turn on TV
2) from the remote press MENU 2,4,7,0 then ENTER
3) TV will state that is INIATIALIZING
4) press the RESET BUTTON on front of TV.
5) hopefully it's fixed.

You will burn up the plastic components in your television if you go with a higher wattage.The cooling system is designed only to handel the specified wattage.Also your set will shut down doe to increased internal temp.I do not reccomend using a higher wattage.You may have dust build up the optics of the light engine or a dirty colorwheel.LG made optics are notorius for getting dirty.Time for a professional cleaning.

My jvc 27" tv was so dark I could hardly see any night scenes, even with the bright all the way on. If you push the sleep button, let go, then hold down the video and display (together), the service menu will come on. Scroll down to #7 (too dark) and adjust the brightness there. Works immediately. Good luck.

Dark picture? hmm.
meaning there is no brightness variation, assuming this is the fault I think U might be having low screen voltage. Check for high value ( around 2W) resistor, preferably from 100K to 680K on the "C" board or neck board. Post back
Rifaa

i have same problem. i replaced lamp but picture is still dark.

there are 2 clocks 1 for lamp and 1 for total, the total you cant change. dark picture, make sure your got same wattage lamp for replacement, I find original Philips lamps the best. if it continues follow manufacture instructions on cleaning mirror and lens.

Just had that problem wrong lamp used orignal lamp fixed problem

Does not seem like a bad tube alone. Could also be that the Video output Transistors Supply Filter capacitor having dried out. It is a 100mfd 160V capacitor, which on replacement will improve the brightness of the immage. And yes, you could also try increasing the heater voltage (Filament Voltage)of the Pictuire tube, this will definitely increase brightness, but will reduce the life of the Picture tube. You gotta think - 10 year old TV, should I repair or exchange and get a new one???