Apart from rebooting your computer as @John_A advised to try above, if same problem and you have Windows, see if the eM Client icon is open on the taskbar. If it is, try @Gary suggestion below from his post in the following thread.
https://forum.emclient.com/t/account-window-not-shown/87433/2
“It may be open off screen”.
“Try again and see if there is a taskbar icon for this. If there is, right-click in the taskbar icon thumbnail, and choose MOVE. Use your keyboard arrow keys to bring it back into view”.