Do you sync your calendar app with a cloud service (like Google Calendar)? If so, log into that cloud service on a computer to determine whether the appointments have been deleted from there as well. If they haven't, then you've most likely hidden them from view (or accidentally deleted that account from your phone).
Or did you remove the app's shortcut from your homescreen? If so, open the app drawer, find the calendar app, press and hold it, then drag it to the homescreen of your choice.
Or are you missing an agenda widget? If so, find an empty space on your homescreen, then press and hold until you see a menu pop up at the bottom of the screen. Select the "widget" option, then scroll through the pop-up list until you find the one you want to add, then finally, drag it to your homescreen.