Depends how they are wired. It's quite possible that one switch has failed in such a way that neither now work.
Springer - do you know how 2-way switching circuits work? Do you have a multimeter?
If the answer to either of those questions is no then you aren't going to be able to diagnose the problem - all you can do is to start replacing the switches.
I take it you've checked for something as obvious as a broken or loose wire at the switches?
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local