You need a Windows 7 repair disk: System Repair Disc - Create
Boot Repair CD, click through to Recovery tools list to open a Command Line, type
SELECT DISK # (for Windows 7 HD)
SELECT PARTITION # (for Windows 7 partition, determine by volume size)
Now return to Recovery Tools list to run Startup Repair up to 3 separate times with reboots as it will attempt to repair and finally write the MBR to Windows 7.
If this fails you need to run full Factory Recovery which might allow you to save your files first.