If it is a flatscreen TV, they have a backlight that shines through the colour LCD allowing you to see the picture. If the backlight is becoming faulty, it can technically be repaired/replaced but it is rarely economical to do so (for the owner at least - some folks make a business of refurbishing faulty devices since they have all the kit).
Do not attempt the repair yourself by opening the back for a look as there will be mains voltage at the power supply.
You should get an easily viewable picture in a normal shaded room at about 50% on the brightness scale, only needing to go a lot higher if the TV is in sunlight. Remember the brighter the picture, the more power the TV will be using.
If you don't have an easily accessible local TV repair place, it would be advisable to start planning to replace it.
EDIT : Oh, and most Councils will have a specific spot in their recycle centres for electronic devices such as this so they can be checked for repair or stripped for the rare materials.