Messages won't load completely on Yosemite iMac
Yesterday, Messages ran perfectly fine and was synched with my iPhone 5, up until I shut down last night. This morning, Messages appears to load, but shows a blank window with the message "No conversation selected." Most of the items in the top menu are grayed out, so I can't access Preferences, New Message…, etc., and none of my prior conversations appear. No changes in configuration, software updates, etc., have occurred since yesterday. Everything is running as expected with Messages on my iPhone.
iMac 24-inch Mid 2007, 2.4 GHz Intel Core 2 Duo, 4 GB RAM
OS X Yosemite 10.10.4, Messages 8.0 (4752)
Thanks in advance for any help you can offer.
Michael
iMac (24-inch Mid 2007)