how do I get my old photos out of iPhoto?

Only some of the library made it through to Photos and the rest appear to have evaporated.

Go to your Pictures Folder (or wherever you have this iPhoto Library and right click upon the icon. From the resulting menu select 'Show Package Contents'. A window will open with the innards of the Library revealed. Your original images are within folder called 'Originals' and can be recovered from there.

You can easily convert your iPhoto library into a Photos one. Launch Photos with the option key held down. From the resulting dialog, choose your iPhoto Library. Photos will create a new Photos library from your iPhoto media.

At this point you will have *2* Photos libraries. I suggest you get Power Photos from Fat Cat software to merge them if you want.

Was this a photos library or an iPhoto library migrated to Photos. Could you have more than one Library?