How to Recover a Missing Calendar in Gmail
One of Gmail's experimental Labs features is an embedded calendar gadget that shows your upcoming appointments next to your messages. If this gadget goes missing, you may have to re-enable it, disable some of your other gadgets or test for problems within your browser or one of its extensions.
1 Enable the Calendar Gadget
If you cannot see the calendar gadget within Gmail, make sure it's enabled. To do this, click the black cog icon above your messages and select "Settings" from the drop-down menu -- open up the "Labs" tab and find the Google Calendar gadget in the list. Use the search box at the top of the list to find the gadget if it isn't immediate visible. The "Enable" radio button should be checked next to the gadget -- if it isn't, select the "Enable" option and then click "Save Changes" to return to the main Gmail interface.
2 Disable Other Gadgets
Not all of the Gmail Labs gadgets work happily together. It's possible that another gadget is blocking or interfering with the calendar gadget, particularly if the other affects the layout of Gmail or also adds a widget to the main inbox screen. To test if this is the case, disable gadgets one by one from the Labs tab of the Gmail settings screen. After each change, Gmail reloads -- as soon as the calendar re-appears on screen, you know you've identified the add-on at fault.
3 Gadget Settings
Depending on how your Gmail screen is configured, the Google Hangouts chat box might obscure the calendar widget. Click the three dots "..." icon next to the Hangouts chat bubble in the lower left corner to display your gadgets. If the widget shows some of your Google Calendars but not the one you're interested in, click "Options" and then "Edit Visible Calendars." The subsequent list of check boxes enables you to choose which of your calendars are shown within Gmail. Click "Back" to return to the gadget.
4 Potential Browser Problems
A bug within your browser or one of the extensions running on top of it could be interfering with the Gmail calendar widget. If you have an alternative browser installed, use it to open up Gmail to test if this is the case. Use your browser's extensions manager to disable any extensions and add-ons that you think might be affecting Gmail -- pay particular attention to recent additions to the list or tools that are designed to interact with Google's email platform in some way.