2000 chevy cav 5 speed manual won't turn over. bat ok, tried boosting, smelled a little exhaust inside

i drained the battery 4 days ago, left lights on overnight. Boosted in the
morning and drove for 4 hours. Driven it every day since, no problems. This
morning started it twice as I was digging out from 60cm of snow and smelled a
little exhaust in the car as my pipe was buried in snow. Tried starting a third
time and no go. Dash lights up, tried boosting and can hear fuel pump load up,
still no go. new starter put in 6 months ago. any suggestions? Thanks. Mark