Most wallets are "connected" to one given network at a time. To view your Ether / Token balances, ensure that you are connected to the appropriate Arbitrum chain. In MetaMask, you can switch networks via the "networks" dropdown. In this dropdown, select your desired network (either Arbitrum One or Arbitrum Nova for our mainnet networks). If your desired network hasn't been added to your wallet yet, you can add it at https://bridge.arbitrum.io/.