you can try and open it with a hex/allen key then take lock out and replace,if it wont open this way then i'd recomend you phone a double glazing repair company and get a quote as its a tricky job to open without damaging your sash.hope this helps,if you need to ask me anything else regarding this fault feel free to e-mail me from my website.
shaun www.sdwupvcrepair.co.uk, March 2008