Right now I checked again, not by double clicking on the wallet, but by using the left side button "send bitcoins" and the funds are available again. I don't know if this is a coincidence but if it happens again I will check if by accessing the wallet this way it solves the problem.
If you click on Send Bitcoins from the Wallet Properties dialog, you will only be spending from the Bitcoin in that specific wallet. If you have other wallets, then you will not be able to spend from those other wallets.
You can also enable Expert mode and use the Coin Control feature when in the Send Bitcoins dialog to choose the coins that you want to spend.