Usually this happens when your Temple can not reach the APIs we use to discover token balances.
This may happen for a number of reasons:
1. This might be a node related issue.
Switch to a different mainnet node and see if it helps.
2. This also might be a browser related issue.
Update your browser and reset the settings. If it does not help try installing Temple wallet extension on a different browser.
3. This may happen due to certain connection settings or firewall.
Check your connection/temporarily turn the firewall off.
4. Your internet provider may be blocking some online resources for whatever reason.
To check if this is the issue try switching to a different provider or turning VPN on.