This could also happen to you. I opened Daedalus Mainnet Wallet (Win10). The synchronization was in progress and the power went out in the apartment. When the power came back on I restarted the Daedalus program and all four of my wallets had disappeared from view.
I chose to restore the wallet. I entered the recovery phrase, it was accepted. I gave the wallet a name and password. Then I clicked “Continue” and got the error message “An error occured”. I was unable to proceed any further. I also noticed that the chain folder had been wiped almost empty. There was only 800 MB, when there should be over 100 GB.
I completely uninstalled Daedalus from windows and reinstalled it. That helped. Unfortunately, it takes several hours for a full synchronization.
Can you recommend lightwallet which support the restoration of Byron legacy wallets, because I want to keep the transfer history of the old wallet available? Soon I’ll have to switch to something lighter. Especially if I now have to start backing up a chain folder.
I would consider exporting the transfer history once and keep it around as a spreasheet or PDF. Or save the list of addresses you ever used and look up the history on cardanoscan.io (or whatever block explorer is the go-to in the future).
Keeping a 100 GB full node wallet synchronised just for history seems a bit much to me.
You could try adalite.io. Yoroi can only restore it’s own wallets, not Daedalus, as far as I can see (and only read-only on desktop).