I have modified MBR from 0x42 to 0x07 in windows 7 to convert dynamic to Basic disk i.e. 0000001C0 01 00 42 to 0000001C0 01 00 07 and now my system is not booting up it is just showing only c ...

If you want to convert a VirtualBox Fixed Disk to a Dynamic Disk or vice versa, follow the instructions mentioned in this guide.

Its Convert to GPT feature enables Windows users to seamlessly switch a disk from MBR to GPT and keep all data and partitions intact on the drive.