Rear Right handside door will not open on my Ford Mondeo Hatchback?

I use a manual key to open the doors of my 1998 Ford Mondeo Hatchbackand just recently I found I cannot open the rear right hand side door either from the inside or out, sometimes lately I also had trouble opening the drivers door but could open 3 doors from the passenger side, 4 doors before this recent problem. We had a new key cut a few months ago but recently found the one we thought was lost and have been using it too, would this cause a problem, any ideas please?.