Have you checked your account settings with regard to the outgoing mail (SMTP) server?
If it's working on one machine, and not the other, the first thing I'd suspect is that either the server name, port number, security type, or login requirements for the SMTP server somehow got messed up.
Compare what they are on the functioning machine and on the "doesn't send" machine and tweak as indicated.
If this doesn't turn out to be the situation it would be helpful to know the respective e-mail clients you're using at a minimum.
Brian AKA Bri the Tech Guy (my website address is in my profile) Windows 10 Home, 64-bit, Version 1709, Build 16299
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall