Perhaps as a result of your reporting your friend spamming your Facebook and Yahoo Mail accounts, Yahoo may have temporarily put a freeze on all incoming mail, until they can sort out your complaint. Just speculation here, but it may have something to do with the sudden absence of any mail messages in both your inbox and spam folder.
You could try contacting Yahoo Mail support
http://help.yahoo.com/fast/help/us/mail/cgi_feedback and explain to them that you are not receiving any mail. They may be able to determine if there is a problem on their end and offer some suggestions, although I should mention that their support can be on the slow side more often than not.