1.) In the standard language, wegen is usually followed by a genitive:
* Die Autobahn wurde wegen eines Unfalls gesperrt. – "The autobahn was closed because of an accident."
The dative case is used if the genitive would be indistinguishable from the nominative in form, which is the case with plural nouns not preceded by an article, determiner, or adjective:
* Die Autobahn wurde wegen Unfällen gesperrt. – "The autobahn was closed because of accidents."
The dative case is also used with pronouns that do not have a genitive form, and if a possessive genitive is preceding the referent of the preposition.
* Er rief wegen etwas Wichtigem an. – "He called because of something important."
* Er rief wegen Peters neuem Auto an. – "He called because of Peter's new car." (→ wegen Peters neuen Autos is possible, but unusual)
Masculine and neuter singular nouns not preceded by an article, determiner, or adjective may take inflectional -(e)s, although this is now quite formal. Personal names never take an ending.
* Er war wegen Fieber(s) verhindert. – "He was unavailable because of a fever."
* Sie ist wegen Anton nach Köln gezogen. – "She moved to Cologne because of Anton."
Personal pronouns and some other pronouns have special contracted forms with wegen:
* meinetwegen, deinetwegen, seinetwegen, ihretwegen, unseretwegen, euretwegen
* dessentwegen, derentwegen, wessentwegen (dated)
2.) In the vernacular, and occasionally in writing, it is common to use the dative case after wegen at all times, whereby all the above peculiarities cease to apply. To some, the genitive may even sound pretentious in a private conversation.
* Die Autobahn wurde wegen einem Unfall gesperrt. – "The autobahn was closed because of an accident."
3.) In very formal usage, wegen may be used as a postposition (always with genitive).
* Die Autobahn wurde eines Unfalls wegen gesperrt. – "The autobahn was closed because of an accident."