In this so unpleasant situation you may have to copy all page content, refresh the page and paste your content back, to make the publish / draft or update button clickable, even to find out that you lost some of your post content, such as tags or images.
Fortunately, this bug can be overcome easily. If your browser, with or without specialized plugins, offers an inspection option, you can right-click on the greyed-out button and select to "Inspect Element". You will see a line of code such as:
<input name="save" type="submit" class="button button-primary button-large disabled" id="publish" accesskey="p" value="Update">
Simply double click this line to make it editable, delete the word "disabled" and hit enter to activate this change. The update button becomes clickable again! Click it and update or publish your post.