Nexus 5 upgraded to 5.1 and mobile data wont work anymore My Nexus 5 upgraded from Android 4.4.2 to 5.1 now it won't connect to mobile data.

How can I get it to connect?
You're not the only one: google support forum issue.

Seems to happen after mobile data has been unused for a while - ie when you come off Wi-Fi.

Comes back if it reconnects to the network.

Workaround 1 - Toggle Airplane mode on and off

Workaround 2 - in Settings -> More -> Cellular Networks -> Preferred Network Type.  Change the setting, then change it back.

Workaround 3 - go out of mobile range (no signal)