Use the command /loadskin default 1 in game to load the default UI. This can resolve some crashing and interface problems, which are often associated with custom UIs.

You can also restore your UI to the default from the character selection screen, or in-game by opening your options menu (alt+o) and choosing the 'Load UI Skin' button on the lower right side on the General tab. All UI's loaded, including the default, will be listed for you to choose from.