In Luke Jesus appeared to the Eleven on the day of the resurrection (Luke 24:33-36); John records that it was Thomas who was not present at this appearance of Jesus before his disciples and that it was one week later that Jesus appeared to the Eleven when Thomas was also present with them (John 20:24,26)
The above comment regarding the apparent contradiction between Luke 24 and John 20 is actually quite problematic in itself; it does give the information contained in those biblical records accurately but then sort of fabricates this contradiction because of an assumption which is not even mentioned in the comment itself, i.e. the assumption that Judas killed himself either immediately following or shortly after his betrayal of Jesus. In order to solve this apparent contradiction it is necessary to include the records about Judas and to clarify when Judas actually did kill himself.
Luke 24:33-36 records that Jesus appeared to the Eleven, and from John we know that Thomas was not with them at this particular time. If Jesus appeared to eleven of the twelve apostles, and if according to John 20:24 Thomas was absent, then it is evident that Judas was alive at that particular time and that he was among the apostles present. Regarding the appearance one week later, it is only mentioned that Jesus appeared to his disciples and that Thomas was present at this time. That Judas was still alive after the resurrection of Jesus and that he was with the apostles is also clear from other records in Scripture where it is mentioned that Jesus appeared after his resurrection to the Twelve (and not, as is erroneously read into those verses, to the Eleven!) and was seen of them (cp. 1 Corinthians 15:5, Acts 1).
In Matthew 27:5 it is said about Judas, "And he cast down the pieces of silver in the temple, and departed, and went and hanged himself." This is always understood to mean that Judas went and killed himself immediately following this event. But there is no mention of such a time element. From Acts 1:18, the words spoken by Peter in the days between the ascension and Pentecost, it appears that Judas only killed himself during that period of time on his own field which he had purchased with the "reward of iniquity" (this reward of iniquity cannot have been the 30 pieces of silver which he had received from the high priests for his betrayal, because he had thrown those into the Temple, according to Matthew 27:5!). Also, we should note that the words translated "hanged himself" can be translated "was choked up in himself", and be understood as describing how he now, apparently feeling sorry in some way, was emotionally under quite some pressure. But, this term does not (and from considering the other passage in Acts 1, it cannot) mean, that Judas then already killed himself by strangling himself ("hanged himself").
The record in Acts 1 mentions at first that Jesus was together with those 12 apostles he had chosen. These were the original 12 apostles, including Judas (cp. Luke 6:13-16). These were obviously together with him until shortly before or perhaps even during his ascension. Only immediately after Jesus' ascension we notice a change in what is happening in the words of the two men (angels) who now spoke to the apostles and addressed them as, "Ye men of Galilee ..." (Acts 1:11) These words are remarkable because Judas came from Kerith in Judea; he was the only one of the original 12 apostles who was not from Galilee. He obviously must have left the place of the ascension right around this time. When the apostles who were left and who then returned to Jerusalem are named in the following record, Judas Iscariot is missing for the first time.
Judas was again in the group of the apostles and disciples during the 40 days in which Jesus showed himself alive. It is quite remarkable to see the great willingness Jesus had to forgive and the great love with which Jesus walked. The promises Jesus made to the apostles included Judas, but it was Judas who was seemingly not willing to receive and acknowledge the forgiveness available to him and to change his life; instead he was caught more and more in self-condemnation and self-pity which finally drove him to the decision to kill himself