In medieval Europe, trial by ordeals were usually not fatal. We should differentiate between the (generally later) witchcraft trials (involving floating or drowning in water) and the more popular ordeals which happened throughout the Middle Ages and across Europe, involving trial by cross, ordeal by fire, water, sacrament or ingestion.

Trial by fire wasn't the trial by fire which occurred in the Persian empires, where you were pretty much guaranteed to die. It wasn't being burnt alive, or something guaranteed to kill, as we might think. Instead, depending on the country, it meant walking across burning hot ploughshares, coals or iron. Was this deadly? No. Depending on the crime, if you didn't recover quickly enough, you could face the general punishment for that crime, which may be death, but a more pressing matter could be the nerve damage and possible infections in the burns. One of the reasons burns are so dangerous because of the risk of infection, and therefore there was most certainly a risk of death when doing trial by fire, due to the infections from burnt feet, as well as possible long lasting disability, if you were not executed for your crime that is.

Trial by water could also cause injuries and deaths a similar way. Trial by water could involve putting your hand in boiling water (ordeal by cauldron) or other hot liquids, reaching a stone (under King Athelstan) or a ring, in some cases (Frankish law). This would scald you severely, and once again could lead to potentially fatal infections, but the trial in and of itself wasn't fatal. Again, depending on the injuries and heat of the liquid, you would certainly be risking permanent disability and scarring.

The ordeal by cross would not be fatal. It tended to be in place of duels, so what we might nowadays sue people for, would, under Early Middle Ages Germanic law, be decided by ordeal by cross. This would involve simply holding your hand out towards a cross until one dropped their hand. This was, obviously, not fatal.

Trial by sacrament meant ingesting the Eucharist (and if you did this while swearing you were innocent, and were guilty, you would either die within the year or be damned forever, or both, depending on the place). Again, not fatal. Trial by ingestion was similar. Of course, there was the famous story of a man who choked to death on cake during an ordeal by ingestion, but this is probably apocryphal, and your chances of death were very slim. Even if you did choke, this did not mean choking to death at all.

So, in general, trials by ordeal were not fatal, or not designed to be, whether you were found to be guilty or innocent. However,, both trial by water and by fire could certainly be fatal due to causing festering wounds and infections, and could also cause scarring and disabilities.

You may be interested in reading The Trial: A History from Socrates to O. J Simpson, which is, of course, very broad, but provides a not bad introduction into the evolution of trial by ordeal in one section.

Other recommendations: The laws of Salian Franks by Katherine Fischer Drew, Law and Society in Early Medieval Europe: Studies in Legal History, The Laws of the Earliest English Kings by Frederick L. Attenborough and Criminal Judgement Through the Ages: From a Divine Judgement to Modern German Legislation.