I don't know exactly what happened as I live with others in an apartment, but ever since this morning, when I flush my toilet, the water goes up very high then drains slowly and weakly and when the water comes back to refill the bowl, it is at a very low level. Anybody know what's wrong and how to fix the problem? I tried plunging it, but to no avail.

Check to be sure the little white tube is connected from the ballcock to the overflow tube.
That supplies the bowl as for the slow flush Look down at the bottom of the bowl. If there is a small hole, then that is a jet that starts the syphon action. If it's clogged the water will just swirl around and slowly go down leaving solids behind. Take your finger,(UGH! ) and run it around the inside of the opening. Over the years minerals build up and cut down on the syphon (flush) action. If it is rough or you feel build up, take a table knife and put a bend in it to get around the curve in the bowl and chip and scrap it clear. Next take a coathanger and clear out the holes around the rim. They start the swirling action. And last, check the water level in the tank. It should be 1/2" below the top of the over flow tube. And speaking of the overflow tube, Make sure the small 1/8" tube from the ballcock to the white overflow tube is connected so it discharges in it and that it's flowing when the ballcock fills. This is what raises the water level in the bowl. For a good solid flush they all have to work together. Hope this helps and thank you for rating my reply.. Tom