This article is from the Sourdough Starters FAQ, by Brian Dixon briandixon at hotmail.com.
No visible bubbles, but you believe you have done nothing that could
have killed the yeast, i.e. the starter has not been subjected to
temperatures in excess of 100 degrees Farenheit or so. It's possible
that you neglected to feed the starter for so long that it appears
that all life has gone out of it. Quite often, starter in this stage
is quite sour. And equally as often, starter in this stage may be
very mild. The starter may have lacto-bacilli growing in it (sour
smell) but the yeast has not taken off yet, or nothing at all is
growing in the flour/water mixture yet.
 
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