Make sure you are using supported software
Trello can't support every version of every browser, so it helps to make sure you're using one that's supported and tested by Trello.
Next, try logging into Trello in the following ways to see if you can reproduce the issue you are having:
Try using a different browser on the same computer
Try using a different computer
Allow cookies from third party sites in case you're running into any issues with linking to GoogleDrive or Dropbox