Can't Open Persistent Store

I was using FCPX literally last night - less than 12 hours ago. This morning, when I tried to open the library I've been using for weeks, it said it couldn't be opened and that "An error occurred while opening the persistent store." I can't seem to get it to work. I can't figure out how to import the events into a new library, and I'm worried it's going to happen again.

System Specs:
OS X El Capitan - 10.11.5
3.5 GHz Intel Core i7
16 GB 1600 MHz DDR3
NVIDIA GeForce GTX 780M 4096 MB

FCPX 10.2.3

Plenty of internal and external hard-drive space.

Update: After having restarted the computer, and a brief panic attack in which the external hard drive wouldn't load at all, the library seems to be working fine. It's probably a connectivity problem with the drive, though I don't know what a "persistent store" is, or why it wasn't working before.