Error 1236 'Could not find first log file name...' (no replies)

I'm new to MySql so let me know what info I can provide that will help.

We have master/slave replication set up but the slave_io_running is NO. Last_IO_Error shows:
Got fatal error 1236 from master when reading data from binary log: 'Could not find first log file name in binary log index file'

The relay_master_log_file shows a file that still exists on master but is a few days older than expire_log_days.

I copied last night's backup of the master to the slave, stopped the slave, dropped the db's on the slave (except for mysql & information_schema) and then executed the .sql script to restore all db's. I reset slave, then started the slave but I'm still getting the same error.

If I'm restoring from scratch, why does it seem to have a problem with a bin log file that's several days back? Or am I completely off course?

Thanks in advance. -tim