On Daedalus 4.0.5 we introduced some changes on how Daedalus fetches and displays exchange rates and wallet value in other currencies. These changes can make the currency conversion disappear from your Daedalus wallet after the upgrade.
To fix the problem, run Daedalus, wait for it to sync and:
1) Go to the Wallet setting Menu
2) Select Wallets
3) Turn the feature off
4) Turn the feature on again
5) Select your currency