It really depends on the fungus. Some of them will give you a bad stomach ache, and some have enough poison to kill you several times over! Some of them even make your brain start to hallucinate and see things that are not there, so it really depends on the species of fungus you eat.
The brightly coloured ones are normally the most poisonous.
I guess it depends on how poisonous is the fungus and the quantity you eat. If the fungus is not very poisonous, you’d get sick, maybe if you eat a lot you get very sick. But if the fungus is very poisonous, you could even die. So you really have to be extremely cautious if you ever pick up wild fungus.
The symptoms you get will depend on the type of poisonous fungus and the quantity you eat. For example, mushroom poisoning. Mushrooms are fungi and contain some toxic substances. The symptoms can vary from slight gastrointestinal upset to death. Some fungi are very deadly if consumed eg false morel, ergot etc