My friend's electrum wasn't working so I installed the latest version and we took his vault electrum.dat out of cold storage and put it in the wallets directory on his linux system.  (There was a file called default_wallet in there that I'd never seen before - did they stop using electrum.dat files at some point?)

Anyway, ran Electrum, opened his electrum.dat as the active wallet and despite having a network connection, the wallet is showing as zero BTC and no transactions are showing.  I opened up the electrum.dat to make sure it was the right public keys and it was.

What am I missing?

Thanks in advance for any help you can provide.