Is this the first time? If so, we would advise to call the employee and if you are unable to contact the employee try their next of kin.

If it is established that they are sick, then a return to work needs to be conducted on their first shift back with a reminder of what the absence management procedure is.

If they do not have a reason for their absence and you were unable to contact them, they need to be invited in to an investigation in line with the procedures in your Company Handbook. Thee information given at this investigation will then dictate if you need to follow a formal process.