Because or because of?

Both because and because of can be used as adverbials of reason. For example:

(1) She cried because she was in pain.
(2) She cried because of pain.

Consequently, a common error is to use because of in the same way as because:

(3) *He missed school because of he was ill.

Sentence (3) is unacceptable because "he was ill" is a clause, but because of is a preposition and should be followed by a noun phrase:

(4) He missed school because of illness.
(5) The flight was delighted because of a storm.

Notice that both illness and a storm are noun phrases.

The word because, on the other hand, is a conjunction. It connects two independent clauses:

(6) He missed school because he was ill
(7) The flight was delayed because there was a storm.

In (6) for example, "He missed school" is a clause (the main clause) and "he was ill" is another clause. The conjunction because joins these two clauses.

If we try to replace because of in (5) with because, the result is also ungrammaticality:

(8) *The flight was delayed because a storm.