This is a query that often comes up during the winter months! gardens often become saturated during periods of persistent long periods of rain, providing the water drains away in a reasonable time, maybe a couple of days or so once it has stopped raining then there is no cause for concern, the problem is when this does not happen and the garden or areas of the garden continue to be water logged, if this situation continues for very long periods of time it can be detrimental to the health of the garden and plants in it. When the air in the soil is forced out by water Oxygen is depleted and the soil will become anaerobic, that is, stagnant, the most obvious sign is it will smell bad!
Sorting this out is not an easy task, clearly the water needs to be drained away and the most effective way to do this is to install some sort of drainage, this may be a bigger task than expected so other, quicker, remedies are considered, often customers have read or been told that adding worms will sort out the problem, this is rarely the case, sometimes if it is a localised soggy area then worms can help. If you suffer from a truly waterlooged garden then adding worms will, at best, result in the worms moving away or at worst, the worms dying off, the only sure way is to put in some sort of drainage system then add the worms , they will then provide the conditions in the soil to allow the drainage system to work properly.