My tachometer guage is not working at all

What is your question?

My tachometer guage all of a sudden went from 3 to 6 while I was driving and it as not fast Now it has gone all the way around to the bottom center and barely moves The "check engine" light did not come on and the car was not racing What is the problem and how can I fix it? I am a Senior on a limited income and would like to know how much to fix it and what is involved Dee