My bathroom sink is clogged, this has never happened before and I’m not sure why what are some common reasons why a sink would clog ?

Get a pail and stick it under the P trap and undo the drain plumbing and wash out in the tub.  That is the accumulation of toothpaste, soap, and whatever else you put down that sink.  Mine was clear jelly  .  Once that pipe is clean, hook it back up and run hot down it for 5 minutes on FULL BLAST TO WASH THE PIPES OUT.  You are going cheap with the water so stuff stays in the pipes.   You want that flushed out, so use more water.  It will keep it cleaner longer and it will never happen again. In the P trap is where you will find your GOLD ring you dropped years ago.

.  Toilet plunger will not work if you got a slit or round holes near the rim.   Most bathroom sinks have that so air blow in and out there as you plunge.

Hair and shaving foam. My sink gets clogged regularly because of shaving foam. I use a toilet plunger and it clears in a few seconds. 

im a builder ..in my time ..over 50 years ..i have unclogged a few hundred sinks ..EVERY one included hair ..

Hair and years of accumulated soap make a excellent plug.

Soap never really dissolves and little by little if creates a mound of slime that clogs the drain pipe.

Hair is merely the rebar that holds it all together

the most common is that the stopper mechanism is completely fouled with human hair (so, remove the stopper actuator mechanism, take stopper out, and clean it over the trash).  second most common is that the trap under the sink [which comes apart without serious tools in most cases] is completely fouled -- usually with hair but sometimes because something was dropped into the sink.  -- grampa

Women wash their hair in whatever sink (lavatory) is available. Add two and two.

Hair is a common reason ...that usually gets hung up on the stopper.

Then there is just **** that usually ends up plugging the "P" trap ...which is the "U" shaped pipe under the sink. Both are easy fixes.

To check the stopper ---see where it attaches to the straight pipe just below the sink basin ..remove the small nut that is attached to the pipe and lift out the stopper.

For the "P" trap... if you have PVC plastic pipe, get yourself a container and put it below the trap.... then remove the two plastic nuts (should be able to do that with your hand) (counterclockwise) and clean out the trap ...its going to be slimy....  

Hair is a common problem.  You can buy a plumber's snake and try that if you don't want to remove the trap.  At the very least the snake can potentially give you an idea of the location of the clog.  

In a bathroom - some common problems includes too much hair in the drain or something falling into the drain and plugging it. (caps from toothpaste or shampoo bottles are common problems.)

The actual problem could also be farther into the pipe and not directly at the sink.

Start by taking off the trap under the sink and seeing if the clog is there.  (the trap is that "S" shaped part of the drain under the sink.)