Electrum 1.x wallets use a different wallet format to Electrum 2.x wallets. When upgrading the new Electrum 2.x software should convert the 1.x wallet file into a 2.x format wallet file. However, after the upgrade is complete the old 1.x software can no longer read the new 2.x format wallet file.
It's best to make a backup of your seed phrase and wallet file before upgrading. Did you backup either of them? You can use backups to restore access to your coins.
These links to the docs explain the wallet formats and upgrade process.
http://docs.electrum.org/en/latest/seedphrase.htmlStarting with version 2.0, Electrum uses the following Seed Version System....
http://docs.electrum.org/en/latest/faq.htmlSome Electrum upgrades will modify the format of your wallet files.
For this reason, it is not recommended to downgrade Electrum to an older version, once you have opened your wallet file with the new version. The older version will not always be able to read the new wallet file.
The following issues should be considered when upgrading Electrum 1.x wallets to Electrum 2.x:
Electrum 2.x will need to regenerate all of your addresses during the upgrade process. Please allow it time to complete, and expect it to take a little longer than usual for Electrum to be ready.
The contents of your wallet file will be replaced with an Electrum 2 wallet. This means Electrum 1.x will no longer be able to use your wallet once the upgrade is complete.
The ‘Addresses’ tab will not show any addresses the first time you launch Electrum 2. This is expected behaviour. Restart Electrum 2 after the upgrade is complete and your addresses will be available.
Offline copies of Electrum will not show the addresses at all because it cannot synchronize with the network. You can force an offline generation of a few addresses by typing the following into the Console: wallet.synchronize(). When it’s complete, restart Electrum and your addresses will once again be available.
If you didn't make backups then your error and some solutions to it are in this thread.
https://bitcointalksearch.org/topic/turned-on-comp-today-and-electrumdat-is-gone-153718The dev says the error means your wallet file is corrupted, and if you haven't got the seed to try opening your wallet file in a text editor to see if the seed's written inside it.
https://bitcointalksearch.org/topic/turned-on-comp-today-and-electrumdat-is-gone-153718