Press the "esc" key when it appears to stop doing anything, it should show the boot stages and list the processes as it starts them with an "OK" by each line, and see where its getting locked up.

If you press a key just at the moment boot is starting, you will get the grub boot-loader, look online for a detailed description of how to edit the text to boot linux in single user mode. 

If you have more than one monitor, it could be getting locked up on a graphics card, try disconnecting one or the other to see if it boots.

You could have hard drive problems.  If Linux detects a failure on the hard drive, I think the kernel attempts to remount the drive in read-only mode, which could lock up the boot.   Some system BIOS has hard drive tests, of you could boot off a utility CD and use smartctl to run the internal hard drive diagnostics.  (look up smartctl) online