If you're in pain and can't walk, firstly I think you need to listen to your body and rest, properly, until it is better.
When you say you've started HM training, do you mean you'd increased your running to HM distance, or have you started training from scratch, or something in between? Have you increased distance or pace, or both? When is your HM?
How are your trainers, could you be running oddly due to them being 'wrong' for your feet, or excessively worn? They say about 500 miles in a pair don't they, but I find when I'm going long distances they don't last as long (eg regularly doing 15+ k wears them more than doing 5kms?)
Are you stretching properly after running?
It sounds like you have some sort of muscle pain, 'pulled a muscle' or otherwise overdone it. Personally I would rest for a few days, see if it improves with the usual RICE treatment (rest, ice. Compression. Elevation) and if not then seek professional help, especially if you are intending to continue training on it. Even a running shop or local gym/pt might be able to advise you?