Does it not work if you slide your finger along that space from left to right? As in: your phone won't unlock? Because if it does unlock, this may be a part of the theme - the designer didn't want to have a button.
If it doesn't work, do you have a way to get into Safe Mode so you can get back to Cydia/into Winterboard to disable that theme? Cause that's the only remedy I know - remove the theme.
Marilyn