MacBook Pro showing prohibitory sign on start up

Since you already tried to reset your PRAM and SMC, I can advise you to try booting to Lion Recovery (use cmd + R on start up). This will let you know if the machine is operating fine hardware and storage wise. If you can boot into your recovery HD with no issues - not only do you know the problem is isolated to the boot volume, but you have Disk Utility and an internet browser to run tests and see what step is next.

More and more Macs are getting updates to run Internet Recovery. Once you have your mac booted, you can then know where the problem lies. Worst case, with a current backup, you could just erase your Mac HD volume and reinstall Lion.

There are a few opions:

• If the HDD is not recognized, you need a hardware repair.
• If the HDD is recognized but the Mac HD is not, you will need to create a new partition and reinstall.
• If the Mac HD is recognized, repair disk and restart to test; if the issue persists, reinstall.

If you don't get what I described here, please say so and I'll extend my answer. But you can start with that.

EDIT:
It seems like Apple has made a support page concerning this issue. I hope this can help you!

In my experience a locked (read-only) disk in OS X means one of two things: An incompatible filesystem (Like Windows NTFS) or a dying drive. Yours is certainly not the former and likely the latter.

First, see if it can be repaired because it's worth a shot. Open Disk Utility from within the Recovery partition that it boots you into. Highlight your drive on the left-side column (You should see two hard drive icons. The top one represents the physical device and the lower one, indented, represents the volume that the computer sees) and then click the "Repair Disk" button on the right-side panel. See what it spits out at you in the log. Basically red text bad, green text good.

If it tells you the disk was repaired (green text): Restart the Mac and see if it boots up correctly. If it doesn't try holding option on boot and selecting your hard drive as the boot device. If you get back into the system successfully, BACK UP YOUR FILES with Time Machine or your preferred method to another disk.

If it tells you the disk couldn't be repaired (red text): Your drive might be failing. If you plug an external drive into the computer, you can use Disk Utility's Restore tab to clone your hard drive to the external drive. Source = your drive and Destination = the external drive (You can drag and drop them from the left column. Note: Make sure the external drive is blank because restoring wipes the destination drive before copying.

Once you've restored you could try booting off the external drive (Option on startup, select external drive) and doing the restore process in reverse.