Countertop is separating from vanity in bathroom. How do I fix this?

Either the coutertop or cabinet warped causing the caulking seal to break. If you can press down on the countertop and it moves it can be reattached. You can try to clean out the old caulk, squeeze new caulk in, then weigh the countertop down to the cabinet or use clamps to hold it down. Let the caulk dry overnight. If it pops back up the its warped beyond repair.

Every one is correct. The first thing to do is stretch a string from one end of counter to the other end and see if the counter is straight. it needs to be your first step. Then use shims as mentioned by others to bring the cabinets up till the counter top is straight or clamp the counter top to pull it down to the strait point. You may need to add a couple small blocks under the counter to the top of the cabinet and then screw threw the blocks into the counter top to pull them tight together but make sure you do not use screws to long to prevent going though the top. you can use small L brackets to screw to along the inside of the top rail of the base cabinet to the counter top and good to clamp down while adding the brackets. It sounds like a lot but fairly easy to do.

Thanks all! Very helpful.

It is a fairy new house (3 years) and this cabinet is barely used as it is for the guest bedroom. Maybe it was just poorly installed?

It could be. Then do shim at bottom to prevent further sagging damage. Maybe the installer missed a shim. If it is just caulk, just recaulk it in place, not worrying about weighing or clamping. If there is a gap between the cabinet and top, jam a piece of something in the space to prevent tinted wood putty from sinking too deep. Then recaulk. It's not worth a new top and the expense of replacing, especially if you have to hire someone, because of the sink and faucet removal needed.