STX addresses (and balances) from the Stacks 1.0 blockchain will not be accessible in the new Explorer until the mainnet launch. Until then, please use the old version of the Explorer to view your Stacks 1.0 STX balance. If you would like to check your STX balance on the Stacks 2.0 testnet, we recommend using the CLI until we provide this feature as part of a future Explorer release.