Welcome to the forum! What model camera are you using? Did you go into the settings and see of anything has changed?
Are you getting alerts from Blue iris or from the camera itself?
ok, its wasnt clear from you original post that you were using blue iris. Have you checked your schedule? is everything set up properly? Is it also possible that you accidentally put a profile on hold. Look at the profile box on the top right of BI with the profile numbers. Is there a red hand symbol in the rectangular box to the left of the numbers or is there a circular arrow or timer?
If you have more than 1 profile/schedule, you need to edit the settings for each.