If you get anything like this in your syslog:
Dec 27 20:35:36 web2 amavis[4216]: (!!)TROUBLE in pre_loop_hook: config: no rules were found! Do you need to run ‘sa-update’?
Dec 27 20:35:36 web2 amavis[4216]: (!)_DIE: Suicide () TROUBLE in pre_loop_hook: config: no rules were found! Do you need to run ‘sa-update’?
Then add amavis to the clamav group and restart amavis to get it working again. Easy to see from the error log, right?
sudo adduser amavis clamav
sudo service amavis restart