Does excommunication prevent me from going to heaven?

The short answer is no. The church has differentiated between anathema which is exclusion from heaven and excommunication which is exclusion from the society of the church. They are not the same. For example Calvin in Institutes (4.12.10)

For when our Saviour promises that what his servants bound on earth should be bound in heaven, (Matth. 18: 18,) he confines the power of binding to the censure of the Church, which does not consign those who are excommunicated to perpetual ruin and damnation, but assures them, when they hear their life and manners condemned, that perpetual damnation will follow if they do not repent. Excommunication differs from anathema in this that the latter completely excluding pardon, dooms and devotes the individual to eternal destruction, whereas the former rather rebukes and animadverts upon his manners; and although it also punishes, it is to bring him to salvation, by forewarning him of his future doom. If it succeeds, reconciliation and restoration to communion are ready to be given. Moreover, anathema is rarely if ever to be used.

Other reformers took the same position. Samuel Rutherford makes the same distinction in his Survey of the Survey of that Summe of Church Discipline (1658) commenting on 1 Cor. 16:22 and 1 Cor. 5, as does James Fraser of Brae, on page 210, in The Lawfulness and Duty or Separation from Corrupt Ministers and Churches (1744). In doing so there were upholding hundreds of years of tradition.

For example, Saint Thomas Aquinas in Summa Theologica addresses the issue of an unjust excommunication in 4.21.4

I answer that, An excommunication may be unjust for two reasons.... Secondly, on the part of the excommunication, through there being no proper cause, or through the sentence being passed without the forms of law being observed. In this case, if the error, on the part of the sentence, be such as to render the sentence void, this has no effect, for there is no excommunication; but if the error does not annul the sentence, this takes effect, and the person excommunicated should humbly submit (which will be credited to him as a merit), and either seek absolution from the person who has excommunicated him, or appeal to a higher judge. If, however, he were to contemn the sentence, he would "ipso facto" sin mortally.

But sometimes it happens that there is sufficient cause on the part of the excommunicator, but not on the part of the excommunicated, as when a man is excommunicated for a crime which he has not committed, but which has been proved against him: in this case, if he submit humbly, the merit of his humility will compensate him for the harm of excommunication.

This separation between anathema and excommunication can be dated back to the Council of Tours (5th century) where it was declared that a usurper of the goods of the Church, desires that after three warnings there be recited in chorus Psalm cviii against him he may fall into the curse of Judas, and "that he may be not only excommunicated, but anathematized, and that he may be stricken by the sword of Heaven." . This distinction was introduced into the canons of the Church soon thereafter.

A formula for this ceremony was drawn up by Pope Zachary (741-52) in the chapter Debent duodecim sacerdotes, Cause xi, quest. iii (which was distinct from excommunication). The notion of separation is further reinforced by John VIII (pope 872-882) who wrote in the decree of Gratian (c. III, q. V, c. XII): "Know that Engeltrude is not only under the ban of excommunication, which separates her from the society of the brethren, but under the anathema, which separates from the body of Christ, which is the Church." (See Catholic Encyclopedia -- anathema)

In 1917 the Code of Canon law dropped the distinction and the term "anathema" was no longer used. In the current day the church defines 3 types of excommunication:

• a jure Ferendæ Sententiæ -- excommunication by law with a finding of guilt by an ecclesiastical court (i.e. a mandatory punishment of excommunication)
• a jure Latæ Sententiæ -- excommunication by law without intervention (the act itself excommunicates one). A declaritory sentence may be given which simply confirms to others that the person is already excommunicated.
• ab homine -- excommunication by act of man, that is excommunication by trial as punishment

Finally, the strongest opinion on excommunication was Luther who rejected the effect of earthly excommunication at all:

Let us now consider the matters which should be treated in the councils, and with which popes, cardinals, bishops, and all learned men should occupy themselves day and night, if they love Christ and His Church. But if they do not do so, the people at large and the temporal powers must do so, without considering the thunders of their excommunications. For an unjust excommunication is better than ten just absolutions, and an unjust absolution is worse than ten just excommunications. Therefore let us rouse ourselves, fellow-Germans, and fear God more than man (Luther's Address To The Nobility of the German Nation, 1520)

Or from his Letter to Pope Leo X, "Excommunications are only external penalties and they do not deprive man of the common spiritual prayers of the Church." Of course it should be understood the Luther was excommunicated at the time and died under excommunicated from the Catholic Church.