1. how old is your big o ring? if old replace it.
2. is your o ring keg lubed? if not grab some and lube it up nice and deep like.
3. did you physically bend the lid? if so, you are as dumb as I am since I accidentally did that once. try to unbend it or try a different lid on that keg to see if it is the lid.
4. place dimes under the feat of the keg lid lever - there should be little rubber feet on them on either side of the lid, when the keg clamp is almost down put some dimes under them, then slide it down so the dimes make the clamp push down harder.
5. once you get that done, crank the gas up to about 40 psi so the keg seals on pressure alone, once it does so(make sure to check for leaks with a soap or star san solution) lower the psi down.
6. tomorrow make sure your co2 tank isn't empty. if it is, make up some excuse to get it refilled and then try steps one through 5 again.