You will only be able to send from that address, if that address still has coins in it.
What has likely happened, is that you've sent from that address (we'll call it AddressA) previously and the "change" from the transaction has been sent back to a change address in your Electrum wallet (AddressB).
When you go to send coins, they all now exist in AddressB instead of AddressA... If you check AddressA on blockchain explorer (like blockchain.info) you will probably see that the current balance is 0. If this is the case, you will need to send some coins back to AddressA first.
Once your AddressA has some coins in it, enable the "coins" tab ("Wallet" -> "Coins") if it isn't already... then on the "Coins" tab, find your AddressA... right click and select "Spend". This will set up an outbound transaction that will use just the coin(s) from AddressA that you selected, so it is guaranteed to send from that Address
NOTE: if you already have coins in AddressA (ie. the "Final Balance" shown on blockchain.info is not zero), then you should be able to find it on the "Coins" tab without having to send coins to AddressA first