Two possible occurences may happen related to this error 1236: either you get the error message like so:
Got fatal error 1236: 'Client requested master to start replication from impossible position' from master when reading data from binary log
Or, no error is given and the Slave_IO_Running is returning no while the Slave_SQL_Running returns yes.
If this occurs, it may be because the master went down unexpectedly (and has since rebooted). Because of this, the slave has not received the command to switch to the next binlog file and is stuck at the previous one.
In other cases, it's possible that no error is displayed, but that the Slave_IO_Running still says no. In this case, the master ...