Windows: https://www.zotero.org/support/word_processor_plugin_troubleshooting#error_or_non-responsive_plugin_buttons

Mac: https://www.zotero.org/support/word_processor_plugin_troubleshooting#word_could_not_communicate_with_zotero_please_ensure_that_zotero_is_open_and_try_again

Or if you're saying you don't see the Zotero tab at all: https://www.zotero.org/support/word_processor_plugin_troubleshooting#zotero_toolbar_doesn_t_appear