My calendar disappeared on my iphone. how do i get it back?

You do not write which calendar you are using. But I assume you're using Apple's Calendar app, in conjunction with iCloud.

Start by checking if Calendar is enabled in iCloud. Go to:

Settings -> Apple ID (Your Name) -> iCloud. Find Calendar in the list and check if it is activated. If not, activate.

If enabled. Simply try disabling, then turn off your iPhone, wait a bit, turn it on again - go back and activate your Calendar. Also, make sure you are connected to WiFi.

If your calendars do not come back, then you can try to restore them. You need to do this from a computer or iPad. It can NOT be done from iPhone.

Read here how:

If you accidentally deleted your calendars, bookmarks, or contacts from iCloud - Apple Support