Several possible solutions, so let's try from the easiest ones and work our way up :-)
[1] Click on File -> Options -> Advanced
In 'Reminders' section check to see if 'Show reminders' is checked.
[2] Start Outlook with '/cleanreminders' command
(Start -> Run -> outlook /cleanreminders)
[3] Start Outlook with '/cleanfreebusy' command
(Start -> Run -> outlook /cleanfreebusy)
If none of this helps you out, let me know.
There are other things we can try, but you may not like them.
Edit:
[4] A corrupted reminder
Now you'll see why this may be the solution you will like if you have many of them :-(
Open Calendar, click on View -> Current View -> Recurring appointments.
Opened each one of them until you get the error "Recurrence data not found".
Delete this one and you should be all good to go.
Edit 2:
[5] Corrupted cache
Are you using Exchange? If so, disable the cached mode and restart Outlook. This will clear the corrupted cache .ost file. Enable cached mode again and restart once more. Outlook will rebuild your cache files from scratch.
[6] Multiple data files
Is it possible that you created more than one data file? Check this by going to Tools -> Options -> Mail setup -> Data Files. It there is another one in addition to your default data file, it's most likely causing this. Oh, and before deleting the not-needed one - Backup! I can not stress this enough.
[7] Outlook with POP3 mailbox
If you are not using Exchange, Outlook will keep all of your reminders in .pst file, as there will be no .ost cache. That may mean the main .pst file is corrupted. Unfortunately, a step-by-step guide will take quite a while for me to write down here, so let's please make sure that you are in fact using POP3 and steps 5 and 6 failed.
[8] If changed from cached exchange mode - check outlook - options - calendar options - delete offline items
1step 1 the box is checked step 2 didnt work Step 3 Didnt work. Thanks for trying – ben950 – 2012-02-24T11:32:20.467
@ben950, don't worry, we'll get to the bottom of this. I edited the answer, please try step 4 :-) – extremko – 2012-02-24T11:43:04.123
1Step 4 i have gone through didnt have an many as i thought but that didnt show up any errors – ben950 – 2012-02-24T12:02:17.420
@ben950, sorry to hear it didn't work. I added some new possible solutions I could think of. Hope one of them will help. – extremko – 2012-02-24T12:14:42.273
1Thankyou Step 5 worked =) must of been corrupt cache. Thanks one again – ben950 – 2012-02-24T12:21:20.847
Yaaay! I'm so glad it worked! Cheers :-) – extremko – 2012-02-24T12:24:30.793