Which version exactly is installed?
You can get this info from the LR menu:
Help >> System Info
You should see something like
Lightroom version: CC 2015.6.1 
License: Creative Cloud
Lightroom version: CC 2015.6.1 [ 1083169 ] Additionally: I turned off the computer last night . When I opened Lightroom this morning the arrow keys were working and after a minute or so they stopped and went back to giving me an alarm bell and not advancing the screen. Very strange.
Well you have the latest version installed. It’s difficult to say what’s going on unless you have used any keyboard modifiers. It’s working OK for me although I’m still on Yosemite and have not upgraded to El Capitan. The only work around is to use the left and right arrows on the toolbar above the filmstrip. If you can’t see these on the toolbar click the drop-down icon to the far right and select navigation from the choices.
Thanks. OK, that works but it's a pain in a couple of ways. One it's clunky and two, I would really like to know why my arrow keys aren't working. perhaps it's worth a call to Adobe?
Sounds like Lightroom is losing system focus. Does the menubar change when this happens? Do you have anything installed beside Lightroom and creative cloud? Any system utilities? Skype running in the background? Anything that pretends to be antivirus software? It is not normal for this to happen on El Capitan.
Turns out we didn't have an arrow problem, we had a human stupidity problem. The latest Imac has rearranged the keyboard a bit and the arrows are different. I was hitting the more or less keys (which is where the arrow keys were on the old keyboard) and wondering why they didn't work.
Well that was an easy solution!
Well maybe for you, but my arrows keys stopped working and that was not the problem.
I found lots of people posted with the same issue and no clear solution, so it seems clear to me this is a real problem. Here is what I tried- most are suggestions from other posts- and what finally fixed it for me.
Current version of cc SW? Yes, not the problem.
Switch to Develop and back to Library- did not fix.
Add Cntrl key as modifier- did not fix. Tried +Alt too.
Hit Esc key- did not fix.
Switch select from Filmstrip to four squares icon- did not fix.
"Select Navigate from pull down menu"- couldn't find that so did not try.
Close and reopen Lr- did not fix.
Unplug and replug keyboard- did not fix.
Try different keyboard- did not fix.
Restart computer- this fixed it for me.
The next step was going to be clearing Preferences, if that failed reinstalling Lr.
Beyond that perhaps try sacrificing a chicken(please start with a KFC bucket, that worked in Major League).
My other suggestion is never put the computer into "Sleep" mode with any Adobe application open. That is crossing the streams, or in a more timely reference electing Donald Trump. Something will happen, but exactly what is difficult to predict. I'm even careful about letting it go into Power Saving mode with apps open.
The next step was going to be clearing Preferences
When the LR user interface starts acting wonky, that should be one of the first things to try. Sprinkling that magic fairy pixel dust fixes a surprising number of "wonky" problems. See http://www.lightroomforums.net/showthread.php?25197-Resetting-the-lightroom-preferences-fi le-updated
The others do less harm. I aways work from least to most impactful Kemosabe.
You can easily save away a copy of the preferences and restore them if it doesn't help.