I got this error and checked my error log. I found a bunch of errors like this:
2019-01-18 09:03:30 [-][-][soundnpevuons07lqg8c911grb][error][yii\base\ErrorException:2] yii\base\ErrorException: fopen(/craft/web/cpresources/725d4d8/jquery-ui.min.js): failed to open stream: No such file or directory in /craft/vendor/yiisoft/yii2/web/Response.php:485.
When researching I found that there might be some write issue errors with the sub folders of the /web/cpresources folder. I cleared out all the sub folders and then tried to log in again. Now it all works.
craft/storage/logs
files and send to support@craftcms.com?