What triggered this? Is this a new phone? Or was it working all this time and suddenly the IMEI vanished?
As lunatic59 said, this could happen if the EFS partition got corrupted during a flash - but shouldn't happen if you didn't unlock the bootloader to flash something.
If you are stock, you could try a factory reset (unlikely it will resolve the issue, but no harm) - your phone will be wiped clean, so backup any important data first.
If your Bootloader is unlocked, you could flash the factory images - it may help. Before doing this though, if your phone still has warranty, use that and get LG to fix it.