You've not given us much to work with. At minimum, please use the guidelines in this Apple article, "How to write a good question" as a start:
To help other members answer your question, give as many details as you can.
- Include your product name and specs such as processor speed, memory, and storage capacity. Please do not include your Serial Number, IMEI, MEID, or other personal information.
- Provide the version numbers of your operating system and relevant applications, for example "iOS 6.0.3" or "iPhoto 9.1.2".
- Describe the problem, and include any details about what seems to cause it.
- List any troubleshooting steps you've already tried, or temporary fixes you've discovered.
IF you are running Messages on the iMac, yours is much newer than the 1998 through 2005 models this forum covers. Once we know your equipment details we can get you moved to a more active and appropriate forum where you can get faster help.