Some times some windows programs can damage grub2 installation on MBR.
Maybe this is what happened to you. Here is something about it.http://www.supergrubdisk.org/wiki/WindowsErasesGrub
If this is your problem it is pretty hard to track down what windows program is causing this(usually anti-virus or manufacturer software like dell or hp custom software pre-built in the shipped windows. Or any other bad behavior program). Google could help, but you would need a lot of time to spend with it.
I don't use windows much, and when I do, sometimes it damages grub, so my fix is simple.
So I burned a SGB disk and always keep it in hand. Once the MBR is messed up all I have to do is boot the SGB disk and boot into lmde then do a
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# grub-install /dev/sda
it doesn't take more than 5 minutes to do all that and its a pretty easy process.
The ISO for SGB can be found at: http://www.supergrubdisk.org/
Make sure you get the SGB2 if you are using grub2(LMDE default)
When you boot from SGB CD you will see a grub-like menu that will let you find (almost)any system installed on your machine.
If you have any problem you can join their channel at IRC( #sgrub at freenode), they were really helpful when I needed.
Like caf4926 said its hard to know what the problem is from what you told, what I told is a fix that should work in almost any case of MBR overwriting problem. Anyway good luck.