It has been a long time since anything so good happened to the Bruins and their fans, and in the end the gift had to be wrapped in a black bow. Amid the excitement and satisfaction surrounding Boston's 5-1 win over the Flyers last night, good for a ticket to the Stanley Cup semifinals for the first time since 1992, the Bruins lost star center Patrice Bergeron early in the third period when he was felled by Flyers right wing Claude Giroux. Looking woozy and glassy-eyed after getting smacked to the ice in the left circle in the Bruins' zone, the 25-year-old Bergeron gained his feet after a few seconds and skated slowly but deliberately to the bench. Without stopping, the dogged center, by far the club's MVP in the postseason, made his way straight to the dressing room — still under his own power — and was not seen for the rest of the night. What's wrong with Bergeron? Virtually everyone in the building asked that question in the wake of his absence. The coaches and players wouldn't offer anything on the subject, to the point, in fact, they sounded as if they were briefed to say nothing, which was exactly the substance of coach Claude Julien's postgame report. "I don't know . . . I have no update,'' said Julien, virtually one hour after the game ended, which also was nearly one hour, 40 minutes after Giroux dropped Bergeron. "Basically, I can't give you m ore than that.''