Samsung Galaxy S5
I use S planner to save all my calendar events.

Today I went into gmail app > Settings > (selected primary email account) > Ticked "Sync Gmail"
Coincidentally, today I went into S planner app and found almost all my events deleted. I am 95% sure that these 2 happenings are related.

Please help me restore my events or at least find a document that has the data for my calendar (including all previous events)..

Unfortunately, I haven't back up my phone in over 3 months, so this is no solution.