WHY DOES THIS HAPPEN
Rotting at the base is not a good sign for any plant. This suggests that your plant has been getting too much water, possibly for a prolonged period of time. Rotting usually starts at the roots, so if it has progressed up to the body of your plant then it has probably been too wet for some time.
Root rot can sadly often be fatal for plants, particularly if it is a severe case, as it's very difficult for a plant to recover from this. Catching root rot early is key to getting the best chances of recovery, so it can be a good idea to look out for additional symptoms of overwatering, some of which may appear before rotting at the base.
Symptoms of overwatering can include:
Browning of the base (which seems to spread over time)
Weakening of the plant (such as squishy instead of firm leaves in succulent-type plants)
Yellowing of leaves
Mushy / discolored roots
It's always a good idea to check the soil before watering again. Exact watering needs differ from plant to plant - some like their soil to dry out entirely in between waterings, and others prefer to stay a little moist (but never excessively wet) at all times. However, if you notice that the soil is very damp and is struggling to dry out properly, this is a sure sign that your plant is being overwatered.
It's super important to ensure that your plant pot has good drainage. Ideally, every plant pot you own should have drainage holes, and you can also consider using 'breathable' pot materials (such as terracotta) to help your plants dry out between waterings.
If your plant's soil doesn't seem to be draining well, you may want to consider using a different type of soil (different mixes drain at different rates), or it may be a sign that your plant is due a repotting.