Do you have an event loop somewhere else in your code?
If not, try this :
QEventLoop loop;
m_NetworkAccessMNGR = new QNetworkAccessManager(this);
QNetworkRequest networkRequest;
networkRequest.setUrl(QUrl("http://www.sample.com/test.php?un=user&pass=123345"));
QNetworkReply *reply = m_NetworkAccessMNGR->get(networkRequest);
connect(reply, SIGNAL(readyRead()),
this, SLOT(slotReadyRead()));
connect(reply, SIGNAL(finished()),
this, SLOT(slotOnRequestCompleted()));
connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
this, SLOT(slotOnRequestError(QNetworkReply::NetworkError)));
connect(reply, SIGNAL(sslErrors(QList<QSslError>)),
this, SLOT(slotSslErrors(QList<QSslError>)));
connect(reply, SIGNAL(finished()), &loop, SLOT(quit()));
loop.exec();
Does that work any better?