Please examine the domain log and the respective node log for clean hints about the reason why the Integration Service wasn't available.
One common reason occurs if an Integration Service is terminated unexpectedly (e.g. by shutting down the server without stopping the Informatica node before doing so). In this case the service process configuration (not to be mixed up with the service configuration!) is disabled and has to be re-enabled manually; only then can the Integration Service be started again. The error message in the domain usually indicates that, but as of my experience sometimes this error message doesn't occur as plainly as it should.