Open Keychain Access in Utilities, use Keychain First Aid under the Keychain Menu item, then either check the Password under that item, change it, or delete it and start over.
You may have multiple entries.
Open Keychain Access in Utilities, enter the part after the @ sign in the search bar, hit enter
My gmail was hacked also. I changed password on computer without a problem but on my iPhone it won't read new password. I was concerned it was a keyboarding issue with using cap/lower case numbers and letters. I am learning to make my passwords more secure but seems like it doesn't work on the iPhone keyboard.