Try the below mentioned steps:
1. Open Outlook->file->Import and Export.
2. Select Export to a file->next.
3. Select Outlook data file (.PST) ->next
4. Select calendar->next.
5. Browse to desktop and save the calendar.
6. Click next->finish.
7. Remove all the calendar entries from Outlook.
8. Click on File ->Import and Export.
9. Import from another program or file-> next.
10. Select Outlook data file (.PST)->next.
11. Click browse and select the calendar from desktop.
12. Click next->finish.
13. Check if you are able to work with calendar entries.
If the above step does not resolve the issue scan the PST file and check if it helps.
http://support.microsoft.com/kb/287497
http://support.microsoft.com/kb/272227
Ruchi Bisht