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.
(1) No one has been able to disprove the existence of God.
(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.