Lots of folks having this issue judging from recent posts here. Me too.
The Option+Command Repair Library solution didn't work. I restored the last previous photo library system file that was working fine prior to El Capitan's release, Got the same "cannot open the library" error message, then tried the recommended Repair option to similar effect. Posted here in another thread and was recommended to trash the whole "old" photolibrary and start anew because everything is already on iCloud. I had been hoping to avoid that "solution" because uploading/downloading from that iCloud Photos library takes forever and a day/week.
I followed the advice, anyway, as I was out of choices (the Time Machine backup file was useless under the El Capitain version pf Photos).
Opened a new instance of Photos and told it to make a new library as recommended.
Then made that the System Library for Photos on this machine (MBA). And connected it to my iCloud account.
From there, it started to update/synch/and download what I had spent months culling in my Photos library.
I knew this was going to have to happen (yet) again and had been hoping to avoid it (yet) again.
Apple has sucked at syncing from as far back as iDisk, MobileMe, and they're still trying to figure out iCloud.
Sure enough, I opened a New Library as recommended, then waited for iCloud to populate the app with my previous stuff. Surprisingly, it went pretty fast on the updating part. Then it bogged down. On my MBA I had chosen "Optimise Storage" to save disk space, and it's been crawling like a 3-year-old across a carpeted floor for the last two days in terms of uploading/downoading.
i know from earlier that the answer is wait and let the Apple servers catch up.
Still, I was using Photos with few problems under Yosemite, and it seems El Capitann broke it for me and others. Not indicting the program itself-indicting it's implantation in El Capitan. There are enough posts here from users to indicate their is a problem.