The most likely explanation is that mining stopped because the chain reached the end of the setup period, the blockchain parameters have mining-requires-peers=true and you are running a single node with no peers.
You can verify this by seeing if blocks matches setupblocks in the output from getinfo and by checking the value of mining-requires-peers in the output from getblockchainparams.
If so, and you don't want to start a new chain, the solution is either to connect another node, or else to issue the stop command, and then restart multichaind with the extra -miningrequirespeers=0 option.
