If it's a slony replication then master continues to work and will catch up when slave is available.
However if the Streaming replication is used, Matser continues to work if slave is unavailable till it finds space for WAL archiving on master, depends on the archieving strategy that implemented while configuring replication. WAL's keep accumulating on the master server till it applies to Slave server after it is available. As long as it find space on master, there is no impact but once it fills all the space, master may go down.
