Windows 7 Not Detecting Display Benq Model Et-0026-n

I reinstalled windows 7 and everything was working fine until today i opened my computer and suddenly screen resolution was changed automatically.

I went to screen resolution and saw in display drop down Benq was not listed it was showing 1- generic non-pnp monitor.

I set my desktop computer screen resolution to benq 1200 x 800 but now since benq is not showing in display i am unable to see 1200 x 800 in resolution.

Before also this happened and when i clicked on detect button in screen resolution it automatically shown benq in display but now i clicked on detect and benq is not shown in display ?

Any specific drivers do i need ? if yes please provide link to it my lcd is 22 benq model et-0026-n I dont have cd/dvd for monitor i may have lost it.