It sounds like what I did a couple of seasons ago playing football, also involving a tackle.
I didn't think much of it straight away, but I got up and I could hardly walk as below the knee felt like jelly. It didn't hurt that much, just had no real feeling of below the knee.
I had stretched the Inner ligaments that go alongside the knee (cant remember the medical name for it) and there isnt much you can do for it. I was told to take anti inflammatories, ice and rest. Once doing that for a week and half or so I started doing exercises on it, luckily I had some latex strips which I had previously used from a previous knee injury and used those as a resistance band.
However I would visit your doctor about it, I wouldn't go to A&E as there isn't much they can do about it and your doctor would give you more time to diagnose the problem.