For Mac, I have found THIS. It might be related to MacOS privacy settings for Megasync.
Open Mac “System Preferences” app, then “Security & Privacy”. Click
“Privacy” at the top. Click “Files and Folders” on the left. On the
right, find MEGAsync.app and toggle the checkbox to UNCHECKED. If it
prompts you to quit Megasync, decline. Then toggle the checkbox back
to CHECKED. When it prompts you to quit Megasync, choose the option to
quit/close Megasync. Now if you restart Megasync, it should start
syncing again. If not, then ensure Megasync Preferences > “Syncs” tab
has the local folder enabled/checked (see first paragraph, above).
I have personally seen this problem on Linux, where the synced directory was on a second drive that was not mounted at startup, or that was mounted after Megasync started. This can be fixed by delaying the startup of the client as said here.