If the file manager doesn’t work, the first thing to do is to try to connect with FileZilla instead. Doing so lets you check whether it’s an issue with the file manager or with the FTP access of your account, and if it’s the latter, you’ll have a lot more log information to help figure out what it is.
Also, if your website is down as well, please note that there is currently an outage going on which breaks FTP and file manager access for some sites.