If you have any window at all then just right click it and make a new one. Otherwise you can try logging and if that doesn't work you may have to delete UI_charactername_servername which will bring it back but will also delete a lot of your settings. And if you have modified your UI a lot then it could be a bit of work to redo. Unless your character is a low level? Once you have it back then also right click and make sure "close on escape" is not checked
You can use copy layout and choose a default one and start your whole UI over, just /copy and a box will pop up with the layouts of other characters and some default ones.
