Arguments from ignorance infer that a proposition is true from the fact that it is not known to be false. Not all arguments of this form are fallacious; if it is known that if the proposition were not true then it would have been disproven, then a valid argument from ignorance may be constructed. In other cases, though, arguments from ignorance are fallacious.
Example
(1) No one has been able to disprove the existence of God.
Therefore:
(2) God exists.
This argument is fallacious because the non-existence of God is perfectly consistent with no one having been able to prove God’s non-existence.