Is the coolant level dropping over a short period/a couple of days?
If so, as it is meant to be a closed system, the coolant is either leaking to air (from a bad joint, split pipe, etc etc) or leaking into the engine block, normally via the head gasket.
If the coolant level is remaining the same, the likely culprit would be the coolant level sender unit/switch being faulty.
We really need more description of what is happening to be able to help further.