There was plenty of medicine in Medieval times that would be more likely to kill you rather than help, but there are a few that would keep you from death.
Commonly in medieval times, urine was used in medicine, and so was animal dung. This, obviously, did not help much. Another common practice was bloodletting; a process where your arm is cut-or leeches are placed- to drain blood (and whatever is making you sick) out of your body. Blood-letting is supposedly what killed Robin Hood too.
While cutting would not help, the leeches are now known to help circulation, so if you are not getting enough circulation to your foot, for example, the leeches might bring back the blood down there and keep you from having to get your foot cut off.
Another useful thing are maggots. If you end up with a wound getting infected, maggots will eat the bad tissue and leave the good. Without them, you might have to get that limb cut off, or you will likely die.
Other than that, there is also plantlife; like Medowsweat or Willowbark that help with pain. There are plants that can promote healing, clotting, and ward off infection.
My suggestion to writers is to look up actual plants or other natural resources available today to use in your novel if you want them to seem more realistic. Often in stories, writers use the “bring the character to the witch to be healed” trick, but that also risks taking the worry out of the novel. If readers know your character can be easily healed, why would they worry about them?
Some writers who use plants found to today also rename the plant and describe it differently, but the way the plant is used remains the same, so it is not an instant magical cure.
I hope this helps!