Posting a picture of windows saying it doesn't work doesn't say much :) In your My Documents UT folder is a Log folder, Launch.log or Launch2.log - if the problem is in UT the log will say so. Whatever has changed in between yesterday and today was prolly it. What did you do? Deleting your folder may work too, but I'd say…
Simply delete your UTGAME FOLDER. Take out any maps you've made first. Happened to me. I re installed it but same problem happened again. Somone here said to just delete your utgame directory and it worked.