iCloud mail suddenly stopped receiving mail. Zigzag line

Suddenly my me.com mail won't work. It shows the zig-zag line next to iCloud. Sending emails works but it doesn't receive emails.

I'm using apple mail, on OS X Lion.

The other account (gmail) is fine.

I can only receive my me.com mail by going into safari & putting in my password each time.

I can't think of anything I've done to cause this.

Rebooting doesn't help.

I'd be grateful for help.