Batman arkham asylum problem please help?

1 months ago i was at the last mission unfortunately my computer has format and batman removed but yesterday i installed it again and i m in the ark ham mission FIRST part where i have Take down the snipers and locate an alternative route to MANSION i pulled the ventilator gate but i cant grapple there no sign of grapple there so please help i located the route but no sign of grapple there and i cannot grapple