iPhotos and mac sierra

If the migration hangs, it may be a problem of insufficient free storage. How much free storage do you have? Photos creates a copy of the library during the migration and needs approx. at least a fourth of the size of the original library as additional storage.

iPhotos was replaced by photos

The upgrade to Sierra is just replacing the iPhoto icon in the Dock by the new Photos icon, but iPhoto should still be installed. If Photos cannot upgrade the iPhoto Library to Photos format, try to repair and rebuild the library in iPhoto before you try again to migrate the iPhoto Library to Photos. The library may have a corruption that Photos cannot fix.

Which version of iPhoto do you have installed? Check in your Applications folder. If the iPhoto icon is crossed out, iPhoto needs to be updated first to iPhoto 9.6.1. It may not be possible for you, if your iPhoto version is not already iPhoto 9.5.1 or an AppStore version of iPhoto.

See Barney's User Tip to update iPhoto:

Can't Update iPhoto because it is not available in the App Store

Once you have again a working copy of iPhoto, run the iPhoto First Aid Tools, one after the other.

• Quit iPhoto, if it’s open.
• Hold down the Command and Option keys as you click the iPhoto icon or the iPhoto Library.
• The “Rebuild Photo Library” dialog with the First Aid items appears.
• Try all four options in turn, starting with the "Repair Permissions".

Make a backup of the library, before you start.

If your iPhoto version does not qualify for updating to iPhoto 9.6.1, try to restore an earlier version of your iPhoto Library from your backup (but do not overwrite your current library, if it contains newer photos)

iPhotos? Are you using iPhoto or Photos?

Where is the library stored? Is it on an external drive or in your Pictures folder? How did you migrate to Sierra?

Did you erase your drive and restore the library from from a backup? Or did you install the upgrade on top of the current system without having to restore the library?