Suddenly I have Photos instead of iPhoto and I'm panicking. What should I do?

I

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Suddenly I have "Photos" instead of iPhoto and I'm panicking.

My Mac was upgraded to Yosemite a few months ago. At that time I had to do something to get iPhoto back in order. That has been working fine since then. Now last night my computer updated apps and I no longer have iPhoto or my iPhoto library. I reluctantly went ahead and opened Photos and it's so different I'm very disappointed and afraid that everything is not even there! I was in the midst of making some photo books and I have old ones I had planned to have reprinted. I don't know if they're still there, intact, or not. I had no warning this was going to happen and am quite upset.

Help!