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Money sent on the wrong network
Your address is the same on every Ethereum-style network. Money sent on another one is still yours, it just isn't on Base.
What happened
An exchange or a friend sent USDC to your address on another network, for example Ethereum or Arbitrum, instead of Base. Your OnlyTokens wallet only uses Base, so that money doesn't show in your balance here.
Nothing is lost: the same key that opens your OnlyTokens wallet controls your address on those networks too.
How to move it
OnlyTokens can't move it for you: we never hold or move anyone's money.
Open Wallet → Export and import your key into a wallet app that supports that network (MetaMask and Rabby do). From there, send it back to your exchange, or move it to Base with a bridge you trust. You'll need a little of that network's gas coin (ETH on Ethereum or Arbitrum) to send it.
Next time
When you withdraw from an exchange, pick USDC and the network named Base. If the exchange doesn't offer Base, use the Swap tab instead.