MDLTR IS MISSING

Hello,
When I swiched on my PC Acer window was apeared, after that black window is apear with a message " MTLDR is missing, press CTRL+ALT+Delete to restart", when I pressed CTRL+ALT+ Delete nothing happens just the same thing....
What I have to do without missing any data from my pc. My PC is contain two parts (C:, D:) the operating system is installed in C:, but I have data in both C:&D:.
Please answer me as soon as possible.
Thank you