Make sure that it's not a hardware issue. This issue is mainly due to the corrupted windows boot manager. To fix this insert your windows installation disk in your DVD-Drive and press F11(Just hold it or keep pressing it) until small dialog box of blue background appears, there you will have to select your boot up device i.e., in this case your dvd drive. After you're done it'll take some time to get you into Windows installation interface. Click Next>Repair your computer(at the bottom left of the window). Select Troubelshoot>Command prompt. Here enter bcdboot.exe C:\windows C: and press Enter.
This should fix it. If this doesn't help try bootrec /fixmbr