I have no experience or knowledge of Canons but it is good advice from Alex. Unless you have a detailed instruction manual and/or are a good camera handyman then I'd be wary about "diving straight in" and attempting a repair by the seat of your pants.
If a new focusing screen can be obtained and is easily fitted then this might be cheaper than a full clean and service of the existing screen from a professional repair company
I hope that it can be fixed and at a reasonable cost.
Mike
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