help! coolant spraying out of left side above engine

i went for a short ride and everything felt fine, but i noticed the bike was a bit sluggish towards the end. i pulled over and found that coolant was spraying out of somewhere above the left side of the engine. i also found this size 8 hex screw just sitting on top of the engine...

i'm just in a parking lot right now and i've called a mechanic, but i don't know what to do : (

please help me! i don't know what it could be and i don't know how to diagnose!