Check number against 2 seperate fields and return the entry that matches
Think you've just got a logic error in your Twig code.  This should work:

{% set userInputtedNumber = 3000 %}

{% set entries = craft.entries({
    section: 'channel',
    numStart: '&lt;=' ~ userInputtedNumber,
    numEnd: '&gt;=' ~ userInputtedNumber,
}) %}