if there is no leader orderer available(in case of crash fault) or your orderering service is down in those cases we receive SERVICE_UNAVAILABLE at client end.
make sure your orderering service is running and there are enough orderer node to maintain the quorum.
if there are n number of faulty orderering node:
in kafka orderering service 3n + 1 orderering node would be require to maintain the quorum. in raft orderering service 2n + 1 would be required