What version of Sia-UI are you running? Also are you using windows? If it is an older version (before V1.0.2) then it is possible that siad crashed due to corrupt files but the UI didn't notice.
Could you try the following: (pulled from http://forum.sia.tech/topic/336/a-sia-ui-error-has-occured-error-sia-closed-unexpected/2)
It could be that a file in your data directory has been corrupted, but as long as you remember your seed you'll be able to access your money. Are you running on Windows, Mac, or Linux?
If you could run the following command from either the Command Line/Terminal and post your results here we can help you get back up and running.
Open a Command Line cmd and cd to your Sia-UI's Sia folder. It will look something like Sia-UI-win32-x64\resources\app\Sia\ . You should see a program called siad there. Run the following command and post the results: siad --sia-directory %APPDATA%/Sia-UI/sia