Calendar Empty

I have an exchange server which also manages my calendar. All of a sudden
all of my entries have been deleted. A friend of mine tried to share his
calendar with mine. He gave me permission to access his and when I opened
Outlook I could see his but my calendar was suddenly empty apart from some
birthdays on january 1st (standard b-days). Even after removing his calendar
mine remained empty.

I've already tried replacing the .pst and .ost files with backed up files
from last week but no joy. I also tried the System Restore option from
Windows but that didn't help either. Anyone have any idea where they
could've gone or should I start all over again?

thank you,

- Robert