All my reminders have disappeared - how can I retrieve them?

I use the reminders app across 2 iMacs, an iPhone and an iPad. Everything has always synced beautifully, until now: all my personal reminders have disappeared (and there were a lot of them). On my home iMac, iPhone and iPad, the reminder icon shows a red circle with '3' in it. When I go to the app, those 3 reminders are tasks from my Outlook Exchange calendar. On my work iMac, the reminder icon shows a red circle with '52' in it - that's roughly the correct number of uncompleted reminders in my list. But when I go to the application, they're not there - all I can see are the 3 Exchange tasks. So where have they gone?? The work iMac is suggesting that they still exist... somewhere. If I log directly into my iCloud account, there's still no sign of them.