What happens to deep sea creatures if they reach the surface?
It varies depending which creature is considered. The faster they are made to lower the pressure and raise the temperature, the worse the effects are. Some animals have a liver, which can help to counteract toxic effects at lower pressures, and some fish at 2000m have an air bladder, so they explode if they are hauled up.

Deep sea fish sometimes arrive at the surface with their inner organs popped out of their mouth. 



They have different lipid types than us, because lipids are packed together differently at high pressures, and lowering the pressure on their lipids can make them malfunction. 

Chemicals can act differently at high and low pressures, so a chemical that can be inert at a low pressure, can produce a different reaction at high pressure, producing gas, precipitating into component chemistries. Urea can be toxic at high pressures and fine at low pressures. 

Proteins fold and change shape under pressure, so deep sea animals can use different proteins that may chainge their shape and function at lower pressure.

Dolphins can get the bends if they are scared back to the surface by deep sea exploration industries, too fast for their bodies to adapt. 

here are some info sources:
http://www.deepseanews.com/2016/03/under-pressure/
https://www.quora.com/Do-deep-sea-animals-experience-decompression-sickness-if-brought-to-the-surface