We have configured marketing cloud connect between Service Cloud and Marketing cloud respectively and is working properly for the past 6 months. However we found out that last June 22, the synching stopped and we are not sure of the cause. Has anyone experienced this before and what are the possible reason that this issue might occur
The most common reason for this, is using a non-exclusive user for the integration. I.e. if same user is a regular admin, and resets password, you can experience the connection breaking, until you clear OAuth tokens and reconnect. Hence best practice is to use a dedicated user - only for the integration.
Zuzannamj has provided a nice guide on how to troubleshoot the connector, where she a.o. provides you with the steps needed to clear out tokens: