iTunes Match syncs up different iTunes music collections and enables users to stream music that they have in one iTunes collection into any devices connected to the iTunes Match service. iTunes Match has offered a consistently reliable service since its launch in November 2011, so it's current lack of action is somewhat surprising. It's unlikely to be related to today's news that rock-gods AC/DC have launched their music on the iTunes Store.
Some Macworld readers report that the service has been offline since yesterday evening, and it's clear that Apple is currently having problems with its servers. Apple's iCloud Support page states that "All services are online", however we are also finding it impossible to stream or download tracks from iTunes Match.