Get flashlight and turn it on to max runtime mode; I'd guesstimate 120 hours left current in it's current batteries, throwing 10 lumens. Should be more then enough in a in a pitch black coffin, it's better then nothing.

Second, use the light to retrieve my cellphone. It's a flip phone, so I don't have to worry about the batteries running out before I run out of air. Attempt to call 911 so that I can have additional help. Emergency services stand a better chance of determining my location better then I can blind inside a box, and if they can locate me, firefighters stand a much higher chance of opening this box from the outside then I do from the inside. If that goes well, try to relax and breathe easy so that the air I have left will last.

If I couldn't get through to emergency services, I'd try to determine thickness of the wood on the top of the coffin as well as how soft it is. If it's thin enough plywood, it might be thin enough to use my knife to try and open a small gap in it. If I can get a gap large enough, if the wood thin enough and the soil thin enough, I'd switch over to the sawblade in my leatherman and attempt cut open the lid, cutting a section large enough for my shoulders to fit through.

If that goes well, once the cutting almost done I'd secure the flashlight and knife first, then lower the cutout section to myself and push the section along with the dirt down towards the bottom of the coffin and then basically continue to do with the rest of the soil.

I'd point out though, trying to dig yourself out is extremely risky. There's a high chance that the dirt would smother you the second the cut was done; it's a dead either way may as well die trying situation though.