Are the keys specific to your windows? In my experience, window keys are not cut individually, so some suitable for your windows may be easily available in DIY stores. You may even find a neighbour or relative can lend you a key that will fit your windows so you can close them immediately.
Nah - I have different keys for all the windows. I have phoned a locksmith (I left a message) but if all else fails I will use my usual tool kit to 'fix' the problem. Masking tape and something else equally inappropriate.