Most of the time it can be recovered. Please read the Lost Accounts guide on how to find it.
This mostly happens with third party security and firewall software that blocks WFP from working correctly. The error will often show up in the daemon.log as : [talpid_core::winnet][ERROR] Initiate WFP transaction: The call timed out while waiting to acquire the transaction lock.
Please make sure you are running Linux kernel version 4.8.0 or newer. Check in a terminal with "uname -a".
If it is below 4.8.0 you can upgrade with "sudo apt-get install linux-generic-hwe-16.04"
After that reboot, and then try connecting with the app again.
This can happen if the Mullvad VPN app can't find the TAP adapter. Solution: close and, re-install the latest Mullvad VPN App. Wait 5 mins (for the connection count to clear) and then try to connect, and see if this solves the issue.
Go into the "Network & Internet settings" > "Change adapter settings" and then right-click the network adapter you use to connect to the internet and select "Properties" then double-click on "Internet Protocol version 4" and set it to "Obtain DNS server address automatically"
One possible fix is to enable Seamless tunnel and also to have Reconnect on wakeup enabled.
The error message "Fatal error during driver installation : failed to create virtual adapter: error 2" or "Fatal error during driver installation. Failed to identify newly added TAP adapter." when installing the Mullvad VPN app. This is commonly caused by firewall / internet security software, for instance Comodo, please disable them and then try installing the app again.