Stanley93

Good morning all!
So i started my car and it turned over fine.. then after running 100% normal for three/five seconds, i heard a.. sound, not a bang but a "bump" while in park. After that my car sounded wayyy deeper. (it is cut after cat) still has both o2 sensors. I gave it a little gas and even in park i could tell that it had less power.. In park it took measurable seconds to get up to 2000 rpm. before it would take no time at all.
I popped the hood and i right away noticed that the oil cap was gone. My theory is that somebody was messing with me, my driver door lock doesnt work so i believe somebody took it. I have no better solution.
Anyone have any advice on what went wrong/how to fix it?
Ps. If in wrong thread sorry! new here.
Thanks!
Stanley