my buick wont start!! Battery still works! Makes turn over noise just dont start!?

[Jordan]

96 buick century! Ran it yesterday started making a knock noise recently when started and would get quieter when engine would warm up! was gonna start running mobil1 high mileage 10w-30 motor oil in it and get the old oil changed out since that maybe was why it was knocking even though i just bought this car from a dealer last week and it was fine then and they had just recently changed the oil! it didn't knock when i bought it but i did put some plain old gasoline in it because i was unaware that i should put good gas in this car and i ran a full can of some seafoam through the shitty gas with a full tank it started to knock when almost all of this gas was ran through i recently started putting the shell v power 91 octane gas through it as it is supposed to help clean out old gunk left by shitty fuels as you continue to use it and now today my car wont start as i had big plans to change the oil today i was just starting it to take it to a mechanic and now i cant even take it there please help!! when i was at autozone buying the oil yesterday the last time i drove the car i was gonna look under the hood and check the oil before i went home i popped it then got out of my car and it started to hail i got back in my car without opening the hood so it was just up a tad bit and i drove home the one mile to my house and then closed it once i got home . it had appeared to have got a little bit wet by the headlights and what not up front but nothing major and some water could have come through the giant buick grill in front so i think this was normal amount of water to be in there but now it wont start since this happened yesterday PLEASE HELP I DONT KNOW MUCH ABOUT CARS!!!