I have eM Client set up in desktop and laptop. But my desktop could not connect.Why?

I have eM Client set up in both my desktop as well as my laptop. Both worked perfectly until recently. My desktop could not connect to my mail server but my laptop works perfectly. Why?   
I only have 2 accounts on each and they both worked well in the second account, but not in my primary account.

If you are using the same setup on both machines, make a backup on the laptop using Menu > File > Backup, then copy the zip file to the desktop and use Menu > File > Restore. That will copy the same working system from one to the other.