"If that is true, how do we know if we as citizens of the U.S. have a right to life? Has it been taken away?"
It's not as simple as that. Most states have laws dealing with end of life decisions and withholding treatment under some circumstances. These laws vary from state to state.
Apparently, Florida law, at least as interpreted by the Florida court system, allows the withholding of nutrition and hydration in such cases.
Other states may not. I don't know about all states.
Even in Texas, President Bush, when Governor, signed a law that is even more liberal than the Florida law. This all comes down to a legal question, at least for the immediate situation.
Regardless of the outcome of this particular case, if you feel that such end of life cases need new law, then you should be working to institute such laws. It's very unlikely that any efforts by the public are going to affect Terri's situation.
Rhetoric is not going to have any effect. The case is in the judicial system now, and the federal judicial system, at that. What they decide will be the end of this story.
If you feel that such end of life decisions are wrong, then it's time to work to change them.