Some significant amount of time has passed between the end of Act 1 and the start of Act 2, because by the start of Act 2 everyone seems very concerned about Hamlet's lunacy. Hamlet must have been putting his "antic disposition on" for enough time to cause alarm amoungst the members of the court -- specifically Claudius, Gertrude, Polonius and Ophelia.
Claudius and Gertrude have called Hamlet's childhood friends back from school (presumably Wittenberg) to try to find out what is wrong with Hamlet and to try to distract him and restore him to his normal nature. Gertrude specifically instructs them to "visit my too much changed son." Claudius asks them to "draw him on to pleasures, and to gather so much as from occasion you may glean" about what afflicts Hamlet now.
Polonius enters the scene claiming to know the cause of Hamlet's changed behavior. Based on a report from Ophelia, Polonius thinks that it is the loss of Ophelia's love that has pushed Hamlet over the edge of sanity. He has, as evidence of this theory, a poorly written love poem that Hamlet sent to Ophelia.
Once Hamlet enters the scene, his behavior is troubling. He 'mistakes' Polonius for a fishmonger; he talks about kissing carrion; he talks nonsense about old people. Polonius doesn't know what to make of any of this, but does remark that "though this be madness, yet there is method in't."
Rosencrantz and Guildenstern make very little headway in discovering the true cause of Hamlet's lunacy. They suggest to Hamlet that perhaps he is crazy because his ambitions to the throne were thwarted, but Hamlet doesn't buy into that and he leaves Ros and Guil as bewildered as they were when they started -- even though Hamlet tells them "I am but mad north-northwest; when the wind is southerly I know a hawk from a handsaw." They don't have any idea what that means!
Everyone Hamlet has contact with is worried about him and questions why Hamlet is such a changed person, but no one has a good answer. It would seem that Hamlet's crazy act is working pretty well to this point.