I used to solve the Rubik's cube using the standard algorithm. Recently, after a long time I tried doing it again. I did revisit the algorithm to solve the Rubik's cube, but I'm stuck in the top yellow layer. There is one mis-positioned yellow piece in the corner. How do I arrive at a top yellow state? Is this a possible configuration or my cube is malformatted ? I'm clueless. I have followed all possible steps that I know.
I'm afraid this is not possible to solve using normal moves.
Two things could have happened:
1. The cube was taken apart or some pieces popped out, and then it was reassembled incorrectly.
2. The cube is loose enough that during scrambling or solving a corner got caught on another piece and twisted in the middle of a move.
In the latter case you can probably just twist the corner back without taking the cube apart. Otherwise you will have to pop out an edge piece next to the corner (turn the U face by 45 degrees first), twist the corner, and reinsert the edge piece. This method of disassembly does not work on modern speed cubes, only on older cubes. To disassemble modern speed cubes you usually need to pop off the cap of a face centre and unscrew the screw on the axle.
If a single piece (either a corner or an edge) is rotated while all the others are correct, then the cube is in an unsolvable state.
This has happened because either two stickers were swapped at some point in time, or the cube was not re-assembled correctly after coming apart.
You must remove the piece and rotate it to get the cube back into balance.
That state is not possible/solvable.
The 2 most likely explanations are:
You took apart the puzzle and assembled it incorrectly.
You twisted a corner. Many speedcubes (Including mine) have a problem in that you can twist corners into a new position, putting it into an unsolvable state. This probably happened while you were scrambling the cube. If you think this is the reason this has happened a to fix it, simply twist the piece into place with your hand.