Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
P0300 isn't enough information to do anything with, unfortunately. It's a random misfire code, but misfires can occur for any number of reasons. You don't have a misfire problem now, though, you have an engine that won't run. The P0300 may or may not be related to that cause, but we can't assume that it is just because it's such a generic code.
So, we start with some solid diagnostics. Does your code reader also read any kind of live data from the vehicle?
OK, that's not enough to work with, so let's sidestep that info, and just move into regular diagnostics.
Fuel pressure and spark are the first two big things to check. Remove a spark plug wire, have someone crank it, and see if you're getting a spark if you hold that wire near ground (or preferably, use a spark tester). Go to the parts store and ask them to borrow a fuel pressure gauge, Autozone and Advance and places like that will loan you one for free. Fuel pressure test port is on top of the engine, fuel pressure should be 55-62psi.
I commonly see fuel pump failure, coil failure, ignition cap and rotor failure, crankshaft position sensor failure. It can be any of these things, or something else entirely, so good diagnostics will save you from buying things you don't need.