My Yahoo! Mail From 4 Days Ago Show Me A Message : "Unusual Activity Detected" ! What Can I do?

That type of notice generally means a 48-hour suspension. Unless there really was some sort of questionable or illegal activity, just wait it out and your e-mail privileges will be restored. Sounds like the message you received from Yahoo was probably clear... just wait 48 hours. Then if you still have a problem, contact Yahoo. In the meantime, do some serious cleaning out of your sent and received mail, esp. your InBox. Make sure there's nothing there that would be considered "unusual" or worthy of suspension.