Today I hit my knee very hard. Its slightly swollen and bruised but its so painful I cant walk. I've been applying ice and elevating it. Any advice?

Time to get checked. One key factor when evaluating patients with lower extremity injuries is their ability to ambulate. If you can't walk on it, it tends to be more severe and therefore worthy of more in-depth work-up. If the injury just happened that rest, ice, and a tincture of time (wait a bit) is important to let acute pain settle and reflex motor weakness to resolve. If still can't walk, get checked.
Compress and see doc. you need to compress and maybe get xrays. i would see a sports medicine doctor and see if you need more testing or just some physiotherapy. contact me if you want recommendations if you live in sydney i have two docs you can see.