Check your receiving address on blockchain.info.
https://blockchain.info/If it shows your address contains the coins from coinbase then try restarting your electrum wallet.
If that doesn't work then try the instructions below. Sometimes a server electrum's connected to gets out of sync with the Bitcoin network. Selecting a different one fixes the problem.
Click tools, then network. A popup box should open. Uncheck "select server automatically" and keep selecting new servers from the list that appears, then clicking the OK button until your balance shows up.
