My iphone 4 calendar entries disappeared after upgrading to ios5. I searched and read and tried the various thing that worked for others (delete icloud from iphone, add it back in, restore from backup etc).
None of it worked.
To recover the data here's what has worked for me:
1 Grab the iphone backup file from the backup in iTunes just before upgrading. Copy it out to a temp folder.
2. Grab an iTunes backup extractor. (google it, adding the word supercrazyawesome - thats the one I tried and it worked for me just fine).
3. Feed the backup file to the extractor and pull the item named iOS near the bottom of the list that appears in the extractor to a new folder.
4. Inside the extracted bunch of files/folders, locate Library/Calendar - there is an sqlite database file inside that folder named Calendar.sqldb. That is where all your calendar data is.
5. Use an SQLite database reader (google it - there's free one available, the first one I tried worked for me). Feed it the Calendar.sqldb file.
6. In the Calendar database, choose the item named "Events" - that's your calendar appointments and notes.
7. Export as CSV.
8. Read the CSV into Excel or another spreadsheet to reconstruct your calendar.
9. Last hurdle: to convert the numeric value for dates into date format, use the Excel formula: =”1/1/2001″ + CELL/60/60/24 + OFFSET/24 where F1 is the cell which has date value in spreadsheet and OFFSET is the timezone offset from UTC (7 for California as of 10/16/2011).
Hope this helps.
(Adding some terms to help those searching for an answer: ipad ipad2 ipod touch itouch ios5 ios 5, calendar update upgrade gone vanished missing disappeared broken bug disaster )