I'm running Linux Mint 13 MATE 64-bit. Everything has been working for several weeks. Yesterday, when I tried to boot up my computer, after the BIOS screen flashes I reach a screen with a black background that reads at the top:

GNU GRUB version1.99-21ubuntu3.4

Then there is a box in which I can select from the following lines:

Linux Mint 13 MATE 64-bit, 3.2.0-31-generic (/dev/sdb2)

Linux Mint 13 MATE 64-bit, 3.2.0-31-generic (/dev/sdb2) -- recovery mode

Previous Linux versions

Memory test (memtest86+)

Memory test (memtest86+, serial console 115200)

At the bottom it reads:

Use the ? and ? keys to select which entry is highlighed. Press enter to boot the selected OS, 'e' to edit the commands before booting or 'c' for a command-line.

I have no idea why it started doing this and, worse, I have no idea how to get out of here. No matter which option I select, I can't get it to boot the OS. If I select either of the first two, it reboots to splash the BIOS and then I'm right back where I started. If I choose "Previous Linux versions" I get essentially the same screen with only two choices (which are the same as the first two choices listed above, Linux 13 MATE and the recovery mode). Again, choosing either one of those results in a reboot. If I try to run either of the memtest options, it reads:

error: unknown command 'linux16',

Press any key to continue...

Then it brings me back to the same screen

Can anyone help me please?

Intel Core i5-2500 ASUS P8Z68-V LX Intel Motherboard G. Skill Ripjaws series F3-12800CL9D-8GBRL (4GB x2) Plextor 128GB M5S Series SSD