Imessages are not being sent from my desktop

Question

have one of the newest imacs... purchased it a week after they became available. ive also got an iphone 5. everything works great with imessage.. can send from my desktop to other iphone users, etc. everything has been working great right up until 4 days ago. messages i tried to send would pop up with a red exclamation point and couldnt be sent. i tried rebooting, disabling the account and re-enabling. nothing seemed to work. i even tried reinstalling the os. still the same. its just this computer... i can function 100% on my macbook air.
im out of ideas and desperately need help.

Answer 1

Please read this whole message before doing anything.
Back up all data.
Quit Messages if it’s running.
Step 1
Hold down the option key and select
Go ▹ Library
from the Finder menu bar. Move the following items from the Library folder to the Trash (either may not exist):
Caches/com.apple.Messages
Caches/com.apple.imfoundation.IMRemoteURLConnectionAgent
Leave the Library folder open. Log out and log back in. Try Messages again. If it works now, stop here. Close the Library folder.
Step 2
If you still have problems, quit Messages again. Go back to the Finder and move the following item from the open Library folder to the Desktop:
Messages
Note: you are not moving the Messages application. You’re moving a folder named “Messages.”
Test. If Messages now works, delete the Messages folder on the Desktop. Otherwise, quit Messages again. Put back the folder you moved, overwriting the newer one that may have been created in its place, and continue.
Step 3
In the Preferences subfolder, there may be several files having names that begin with any of the following strings:
com.apple.iChat
com.apple.imdsmsrecordstore
com.apple.imessage
com.apple.imservice
Move them all to the Desktop. There may also be a file with the name "com.apple.imagent.plist". Move that to the Trash.
Also in the Preferences folder, there's a subfolder named "ByHost". Open it and do the same thing.
Log out and log back in. Test again. This time Messages should perform normally, but your settings will be lost. You may be able to put back some of the files you moved to the Desktop in this step. Relaunch and test after each one. Eventually you should find one or more that causes Messages to malfunction. Delete those files and recreate whatever settings they contained.
If the issue is still not resolved, quit Messages again and put all the items you moved to the Desktop back where they were. You don’t need to replace the items you moved to the Trash. Stop here and post again.
If you later decide that you don’t like the results of steps 2 and 3, you can undo them completely by quitting Messages and restoring the items you deleted in those steps from your backup.