The leading cause for a language to become extinct is because the last community of speakers winds up bilingual and shifts to the other language over several generations. Situations that cause this include conquest, cultural assimilation, emigration, globalization, education, increase in global communication and the growing prevalence and power of "commerce" languages (i.e. English, Spanish, Chinese, etc).
There is a difference between dead languages and extinct languages. Dead languages are not spoken fluently in everyday conversation, but might have specific applications in particular schools of thought. Common Western examples might be Greek and Latin in scientific and medical fields. Sanskrit is sometimes thought to be such a language, used only in spiritual, religious or philosophical matters surrounding yoga, Hinduism, Buddhism and Jainism. However, a turn-of-the-century Indian census study reported over 14,000 fluent and active speakers, with attempts to grow the language.
Scholars in the field of linguistics often say that as many as twenty-thousand languages existed around ten millennia ago, yet that number is down to about a third nowadays. Expectations exist that in a decade, the number will go down by half, leaving around three thousand active, living languages around the globe.