iPhoto reset

I have made a complete hash of iPhoto, succeeding in corrupting a lot of the picture files and getting the grey square of doom on many of them.
Fortunately I still have most of the pictures on SD cards and/or CD's.
I have wasted a lot of time recently trying to solve the problem to no avail.
I now want to delete everything to the original "out of the box" empty state so that I can start again.
Can you help. please?
I am clearly almost computer illiterate so would appreciate very simple instructions!

Thank you

Jim

Do you have them copied in iCloud or on a back up disk?

Have you tried this as a repair? How to use the Photos Repair Library on your Mac

Otherwise, delete all, follow instructions here. I would go Photos > Library > Select All > Delete

@silver jim, are we talking iPhoto or Photos? What version of macOS are you running?

The easiest way to start over is to open the app (iPhoto or Photos) and then click on the name on the top men bar and preferences. On the General tab will be the library location, which will be needed later on. Now close iPhoto or Photos and start with the Option Key held down and you get a menu window that will allow you to pick which library you want to use, or more important, to start a NEW library. Start a new library, name it so you know what the new name is and go ahead and import all your images again. One tip is that in the import process you will be given an option of copying the items to the library or leave them in place on the drive. I recommend copying them into the library. I suspect what you did to get the grey boxes was to NOT copy them into the library, then moved the originals to some other place and now the app doesn't know what you did with them. Copying into the library puts the images into a database within the application so that you can then delete the file from the drive, or remove the memory cards, or external drives and still have the images in the database.

Anyway, once you have the images imported and are happy with the setup, go to the location of the old library that you got in the first step and delete the old library to recover the space on the drive.