Follow these instructions to get to a command prompt (and backup your files if needed). Use your Windows 7 install DVD:
Check the letters of your drives (C:\, D:\ etc.) from Notepad's "Open File" dialog.
First make sure your C:\ partition is marked as active. Use the following commands:
(you are now inside DISKPART>)
select disk 0 (select your hard drive)
select partition 1 (select the partition with your Windows installation)
You can now try to restart. If it is not fixed, try to repair your bootloader. Open again the command prompt and change directory to E:\boot, where E: is your Windows 7 install DVD:
cd /d E:\
Run the following command, where C: is your System partition:
bootsect.exe /nt60 C:
Then restart your computer.