This idea requires a bit of mental preparation because you want to act as if what you are about to say is true. And it requires the most solemn tone. Start out by saying you had prepared to speak on topic ‘X’ tonight, but in light of the news you have just received it is no longer relevant. Explain that you had learnt from God himself that the last person to accept him as saviour has done so as of a few hours ago (give specific time if you want). Explain that within a few minutes/hours God will be coming to take his children home, and express what joy it is knowing that soon we will be with him, etc. Dress it up as you see fit. Then focus on the sorrow you have because not all of (assuming you have unsaved children in your youth group) them will be going. Explain (almost apologetically) that they had heard often of Jesus’ love, and how he wanted to save them, but now it is too late. By the time you finish with the sorrowful aspect of Christ’s return make sure that there is a realised finality. That there last chance is gone, and they are doomed.
Dramatic pause here is good as it sinks in. After about half a minute, begin to explain that what you had just said was not true… however that there is coming a day when Christ WILL return, which could be in the next minute, or day, or week, or year. Explain that NOW is the accepting time. And you have opened for yourself the doors to a wonderful presentation of the gospel.