Depends. It all depends on what the medication is for. It may cause worsening of the disease, exacerbation of other diseases, infection, change in function/energy/mental state, organ damage (e.g. Heart attack), or death. If you're on a med, don't stop it without clear instructions from your doctor. Some meds cannot be stopped abruptly due to side effects, sometimes serious.
Depends on med, dise. Depending on the medicine and the disease anything from not much, to progression of the problem without symptoms until it gets worse and worse until you notice something is wrong with you, to you die sooner rather than later, peacefully or horribly. Talk to your doctor.