Your phone may be having trouble reading the SD card. Here's a couple of things you can try...1. Close all apps, kill them if you can, then reboot.
2. If that doesn't work, look in Settings>Storage. If your SD card (external storage) doesn't appear, than it's either an issue with the kernel, ( happens with custom ROMs not being flashed properly, or a code issue) or the SD card itself.
3. To check if it's the card, power off your phone. Wiggle off the "tail" cover where you can access the sd card. Take it out & stick it in a sd card adapter, then into your computer. Your computer should "see" it as a removable drive and read the contents. If your computer isn't reading it, then you can get a Geek squad tech at Best Buys to transfer your stuff to a new card. Buy a good one. Sandisk makes very good ones.
4. If any of that doesn't help, try the most recent nandroid backup you made. Maybe you "zigged instead of zagged" with some of the settings.
These are just some ideas to try.
This was posted by my One V running Rhythmic ROM v1.4 JB. with a 16 GB Sandisk class 6 SD card. No problems!