just some items:

check if password and ftp adress and so on is correct.

try to connect with another program with same credentials to your host see if that works.

check with the host if they have changed something.

ask a new password at your host and try that.

Similar situation here (ver 2020.1.8):

I was working on my page, uploaded to the server several times - no problem.

Suddenly, after one more change, couldn't connect!

These are messages I'm getting:

Started as usual -

It was sitting on it for some time and then:

I can manually connect to the hosting seerver without any issues.

After number of tries and reboots - the program connected to the server.

Happens once in a while, though...

To check it is not a problem with your ISP can you try a different internet connection, perhaps hotspot from your mobile?

If the problem persists perhaps temporarily uninstall any 3rd party anti virus, malware, firewall software and disable VPN if you use one.