It's hard to definitively answer this without seeing a picture of your gradebook (without names of course) and where the 100 is coming from. I can only offer some suggestions of things to look for.
Canvas does not show any grade at all until something is graded. Canvas only includes assignments with a grade in the calculations, so as soon as there is one grade, that will count for all of the grade for the entire course.
The first thing I would do is look through the gradebook and try to see where the grade is coming from. There is likely a grade there, even if you didn't put it in directly. Knowing where the grade is coming from might be helpful in tracking down why it's there.
Do you have any external tools set up? For example, if you use an external homework system, then it could be putting grades into Canvas for you without you thinking about it.
Have you created a missing or late submission policy for your class? It tells Canvas to automatically assign a grade when the due date passes. There is some confusion between the Missing Submission Policy and the Late Submission Policy. The missing submission policy takes a percentage of the grade to award when the due date passes without a submission. A 100% here would mean that you want the student to receive 100% of the grade for not submitting anything. The late submission policy takes the percent of the grade to deduct (rather than award). A 100% here would mean to deduct 100% and the student would get a 0.
Both policies apply to all assignments including those that have already passed. These lessons from the Canvas Instructor Guide explains more about the policies.