I wired a switch to control a light fixture. I also replaced an old fashion outlet with a three prong grounded. Now I have a fuse breaker (the fuses screw in like light bulbs). The first time I turned on the main breaker it sparked and blew a fuse. So I changed the fuse and checked all contacts to make sure no wires crossed or were exposed. I tried it again and now there's no power and all fuses are still good

Hard to answer this with no concept of how you wired them. Tell us how the wires run, colors and was the power through the switch or from the box for the light.

The light was a pull string light. I disconnected the light ran the power line down the wall to a switch. The problem is I don't know if the black and white wires are right. They are old and the black and white are separate wires both with black insulation and no ground wire. The landlord had spliced a piece of black and white wires to the ends to tell which is which. I took the main power line from light spliced it to a 12/2 which I hooked up to the switch, I grounded it on the junction box for light. I also replaced an outlet which had the ability to receive four plugs in one an old fashion outlet. This I also just matched white to white and black to black, but it was a three prong outlet so I also had to add a ground which I also grounded to the outlet junction box. Do you think maybe the wires from the light fixture could've been reversed? Could that stop the flow of electricity throughout the entire apt?

You should not be doing this because,
1. I do not wish to be unkind, but it is obvious that you do not know what you are doing,
2. You said "landlord" so the building does not belong to you,
3. You "apt" so I assume it is a multi-dwelling structure.

You are putting yourself and others at risk. If a fire occurs you could be held liable, even if it is not the result of something you did.
What you are attempting do do is fairly simple but apparently the wiring is quite old and the risk is quite high.
Do you want to be held liable, possible criminally liable, if a fire or something else occurs?

I understand what you are saying, but I do have training I went to school for electrical construction for approx. 2 yrs. So I do if anything have a basic understanding of what I'm doing, it's just that it's been over a decade and I did not certify. But as for the landlord I'm trying to correct his mistake. He would not get a professional to do the minor work of switch and outlet. They marked the wires as to which is hot and neutral, because these were not color coded. So all I did was take a line of 12/2 and spliced it together by their colors and added a switch. But now all other power is off in the one apartment. I checked everything made sure no wires crossed or were exposed nothing. I even unhooked the switch and put everything the same way it was before and still no power. If I could afford an electrician myself I would have done so. But I do understand the consequences of faulty wiring.

That is exactly what I'm referring to. The way I am reading that, you are saying you connect the 21-2 cable to the existing incoming wire by color code. I don't know what you connected the light fixture to.
Proper connections would be to connect the incoming neutral (white) to the light fixture. Connect the incoming hot (black) to the white of the 12-2 cable to the switch (after re-designating it a hot conductor by coloring both ends some color other than white). Connect the black of the 12-2 from the switch to the black of the light fixture.
The logic is that the power (hot and neutral) comes to the fixture box. Neutral is connected directly to the light. Hot goes to the switch on one wire (white re-designated as a hot conductor) goes through the switch (when switch is on) and returns to the light fixture itself.

It appears from your post that you have a load center with a master breaker and fuses for individual circuits. If you have put all wires back the way they were you should be able to restore power.
To trouble shoot the problem you really need a meter. Double check the breaker by turning off and then back on. Check the fuse for the circuit. Some times in the old load centers fuses don't screw in well enough to make good contact. Remove and then screw in again. You need a meter to see if you have power coming out of the load side of the fuse socket.

I would like to thank you for your help. It was something so simple, it was two fuses. The inside is shaped like an upside down L. well the bottom part barely visible was blown on both. Now everything is working.