Nexus 5 IMEI number gone after updating to Lollipop My IMEI number has disappeared on my Nexus 5 and now I can neither call nor text.

Status of the phone says:


Network:
Unknown
Mobile network type
Unknown
Service State:
Out of service
Mobile network state:
Disconnected
My phone number:
Unknown
IMEI:
Unknown
IMEI SV:
Unknown


All of this happened after updating to Lollipop using bootloader. 

I have tried several tutorial videos, such as:


"How to Unroot Nexus 5 on Mac! - Restore to Complete Stock", 
"How to Upgrade, Downgrade, Restore ANY NEXUS Device on a Mac - Easiest Method"


Now is my Nexus downgraded to Android 4.4 and I can't text or call anyone. 
When I try to open the Camera I get the message "Unfortunately camera has stopped". The same thing happens with Google Play Store. 

When I try to take a screenshot I get the message "Couldn't save screenshot. Storage may be in use." It's like it can't connect to the storage.

Can I fix this? Is it possible to restore the IMEI without any backup?

If not, should I lock the bootloader and send it to my carrier (I live in Sweden)?

Note: I have the IMEI number written down.
There is nothing to worry. Please follow the steps mentioned in this Android Enthusiasts answer.

You just have to flash the images properly. Do a factory reset using the recovery mode.


Volume Down + Power
Bootloader screen will be visible
Go to Recovery
Power + Volume up key (few seconds) and then let go of the Volume up key, but keep pressing Power
Select Wipe Dalvik cache
Select Factory reset