help what do I do when The iCloud IMAP server “p03-imap.mail.me.com” rejected the password for user “ at my mac.com account”

How do I find out what password is for the icloud IMAP server. My email account, on icloud opens with my password, but my mail icon on my dock email is stalled and I have not recieved any mail since yesterday on it. I have gone to preferences and typed in my email password and this does nothing. I cannot send out mail. Please help.