It depends on the circumstances. Did you manually turn it off or did it turn off automatically?

If you turned them off MANUALLY, log in to the desktop version of the site. I think you can choose whether or not your IMs are turned on in the settings available in the chat room. You get to that menu when you click the wrench button--something you'll see in the top right hand corner when you're in one of the desktop version's chat room.

If it was turned off AUTOMATICALLY, you might be out of luck. Sometimes the site will automatically shut down a user's IMs if the algorithms flag the user as a spam account. At least in my experience, this has been more common if the user is someone with a lot of accounts or who just carpet bombs everyone with messages, but sometimes it does it to people for no reason. Your best option in this case is to delete the account and make a new one.