Could you please send me a Lightroom diagnostig log - best as a private message with a downloadable dropbox link?
You can trigger the log via LR preferences->Lightromm Mobile...
and when you hold down the alt-key you will notice an additional "Generate Diagnostig log" button.
We’ve identified a shift in OS 10.10 behavior that seems to be tripping us up and are testing a fix to roll into our next release. Unfortunately the only workaround until then is to downgrade to the previous version of Lightroom.
Here I had the same problem, since yesterday. Switching to another catalog didn't help, but a simple system reboot did: now it works again.
Lightroom version: CC 2015.1 [ 1025654 ]
License: Creative Cloud
Operating system: Mac OS 10
Version: 10.10 
Application architecture: x64
Logical processor count: 8
Processor speed: 3,4 GHz
Built-in memory: 32.768,0 MB
Real memory available to Lightroom: 32.768,0 MB
Real memory used by Lightroom: 242,0 MB (0,7%)
Virtual memory used by Lightroom: 940,7 MB
Memory cache size: 0,0 MB
Maximum thread count used by Camera Raw: 4
Camera Raw SIMD optimization: SSE2,AVX
Displays: 1) 2560x1440