Can I fix this lawnmower?

My lawnmower has been working fine for the past couple of years. Well, my dad's lawnmower has. I thought it was time to sharpen the lawnmower blade, so not knowing any better I tilted the lawnmower on its side with the air filter facing down. After sharpening the blade, and hosing the lawnmower off, and washing the air filter with water, I took it outside to start it up. But when it started it made a loud clunking sound, and I thought that the air filter didn't dry off enough. So I took the cover off, and let the air filter dry in the sun for a couple hours, then when I went back to start it up again, it made that clanking sound again. What did I do wrong? What's broken? and Can I fix it on my own? It is low on oil, does it need new oil?