You can connect any number of Ethereum addresses to a single LabelHash.com account (see Understanding LabelHash Authentication).
To sign a transaction from a specific address you must use your wallet provider. LabelHash pushes utilisation of wallet providers that implement the WalletConnect protocol.
Whilst you may have previously connected an address to your LabelHash account, we require an active connection to your wallet provider to be able to sign transactions.
There will be an indicator next to your currently active address in the 'Your addresses' section visible at the top of each page.
If you attempt to complete an action that requires an active connection to your wallet provider you will see something similar to the below:
If you do not have an active wallet connection simply click 'Connect' from the LabelHash header. You can do this at any time from any screen.