Many glitches and issues with Mendeley can be solved by completely shutting down the Mendeley Desktop software or Mendeley Reference Manager, and then reopening it. Make sure to do the same for Microsoft Word if you are using the two in tandem.
If you are having an issue with the browser Mendeley, try logging out and logging back in. Alternatively, try logging in from a different web browser.
If this doesn't work, restart your computer and then try the above options again.
To fix issues with the Word Plugin, try uninstalling and reinstalling it. Here's how: