Several things can match that vague description. check out www.bicycletutor.com, www.parktool.com/repair or www.sheldonbrown.com for more hints and help on how to fix it.
You haven't shifted it!
When we have this problem, we turn the ignition off and shift into neutral, restart the engine and it can then be shifted into drive or reverse.
Loose hold down clamp or broken shift cable.
The nameplate shifted to the right.
The most likely possibility is that your derailleur is out of tune. When a derailleur is out of tune the usual suspects are improperly adjusted screws that control the maximum and minimum settings of your derailleur. If your high gear screw is out of place then you will most likely have the problem you have described. However, do not worry as this is a common problem with a bike that has been used a lot without proper maintenance. Any bike shop should be able to tune your derailleur to near-new settings. They will also perform regular maintenance such as truing tires, checking break pads and some other parts on your bike. If your problem is more severe than what i have described the bike shop will spot the problem and then proceed to fix it. I would suggest asking them to call you before they make any decisions about swapping or replacing parts. This may save you from having to explain that you didn't want to have your derailleur replaced with the 80 plus dollar top-of-the-line replacement. Hope this helps, Zilduli
a controlled shift register is a register that is shifted and controlled..lol
The past tense is shifted.
Shifted is the past participle.
A shift pattern is exactly what it sounds like it would be. A shift pattern is a pattern that moves or can be shifted.
A bit shift is a bitwise operation in which the bits in a value are shifted left or right.
The adjective shift- adjectives shifted around both sides of the noun.
Radiation from a source moving away from the observer is red-shifted. Radiation from a source moving towards the observer is blue-shifted.