iCloud Issues- perhaps

For over a month, "calendar," ex-iCal has not synced between the cloud server and my MacBook Pro (MBP). It does sync with my mobile devices-just not with the laptop. Now, "messages" no longer works on the MBP but does work on the mobile devices. My hunch is that there is an issue with the iCloud and MBP connection.

Even my local computer consultant, a well-respected, professional, business-oriented consulting firm cannot figure out the problem.

What suggestions do you have for my working toward solution, please?