Can't access my photos anymore from iphoto since updating to El Captain
To use iPhoto on El Capitan, update iPhoto to version 9.6.1.
if you did not update before the El Capitan upgrade, see: Barney's User Tip: Can't Update iPhoto because it is not available in the App Store##
The upgrade to El Capitan just removes the iPhoto icon from the Dock. Have you looked in your Applications folder, if iPhoto is still there?
photos didn't merge to Photo app, how do I restore them? Will a different operating system fix this?
You can open the iPhoto Library in Photos by dragging the iPhoto Library onto the Photos icon in the Dock.
It requires, that the library is on a compatible drive. The library needs to be on a locally mounted drive, not a NAS, and the file system needs to be MacOS Extended (Journaled). The drive must not be a Time Machine drive.
Which iPhoto version created your iPhoto Library. If the iPhoto version is iPhoto 7.1.2 or older, you have to prepare the iPhoto Library with iPhoto Library Upgrader. iPhoto '11: About the Library Upgrader