Why I cannot see the image I want to shoot in my camera screen?

Some of these Canon cameras of the era have problematic Sony CCDs which die due to the seal breaking; it bit a LOT of other cameras Canon did not cover: https://canoncanada.custhelp.com/app/ans.... It only covered the ones which gave them bad PR; everyone else got hosed.

If the sensor is black in the final image, you got bit by the Sony CCD defect. The problem is UNLESS you can get a donor with a calibrated lens attached, the fix is very difficult to pull off (and you may need to calibrate the new sensor as well) and expensive to the point it totals the camera out. Plus the part is almost certainly EOL, so all you have is a pool of risky used sensors.

@emiliacortes this "camera works but nothing can be seen" sounds like a LCD issue. A failure of the CCD would also affect the images. Here is what Sony says for the troubleshooting

You can go ahead and check the board itself to see if you have to proper voltages

and here are the voltages you would expect

I know that all of this sounds really complicated and boring. I would go ahead and start simply by replacing the LCD screen. Use this document LCD Replacment to replace the LCD.

While you work on your camera, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair ir War on Entropy!