I converted MBR to GPT and I lost everything on both of the partitions, though I just converted one partition (Bootcamp's) but it still converted them both.
Any ideas how to get my HDD MBR again and reinstall my Mac OS?
You have a misconception on how partitioning works. Both the GPT and the MBR are areas at the beginning of the disk that will hold information about partitions and filesystems. The GPT is a more recent and advanced standard that overcomes a few limitations of the MBR (like raising the size limit from 2 TB to 8 ZB).
There's a single MBR (or GPT) per disk, not per partition. When you change the contents of a disk GPT or MBR, you are in fact changing the layout of the partitions of the disk. Although it is possible to convert from MBR to GPT without losing your data(*), that is not a guaranteed fact. If you convert a disk from a format to the other, you are erasing and rewriting all the information about the partitions that hold all your data. Performing a backup prior to that is highly advised.
I would recommend you stop any writing activity on that disk and run a data recovery software such as Recuva to try to salvage your data.
That Brazilian Guy's answer is basically correct; however, I want to add that Macs use a dangerous cross-breed of GPT and MBR known as a hybrid MBR when setting up a dual-boot between OS X and Windows. I suspect that Aden jason became confused by this fact; a hybrid MBR is likely to contain just a subset of partitions, and the MBR side is used only in Windows. If the goal was to stop using Windows, the procedure to stop using a hybrid MBR would be to replace the hybrid MBR with a standard protective MBR. This can be done with a number of tools; but as an example, in
Moving forward, booting the OS X installer disc should enable re-installing OS X. I don't recall the details of the OS X installer's partitioning tools, but there should be an option to use the whole disk. Use that option and it'll be done. If the problem is that your Mac is new enough that it didn't come with an installer disc, you'll need to contact Apple to get one.
If the disk contained important data that you want to recover, I'd like to reiterate That Brazilian Guy's advice to stop using the disk and either use data-recovery software on it or take it to a data recovery expert. It's likely that TestDisk will be able to recover the original partitions, but I can't make any promises about that. I've never used the Recuva program recommended by That Brazilian Guy, although it appears to be for Windows. (TestDisk is available for Linux, Windows, and OS X.)
lets take this down a notch.
Do you have a TM backup?!?::
EDIT: or hold "option" at startup to boot to USB or External Device.