There are several different reasons that cause the printed object to warp – usually, the source of the problem is related to temperatures.
What’s happening when an object starts to warp? When the object starts to cool down, various forces start affecting the material and they can lead even to minor shrinking. If this force exceeds the one that’s holding the object on the print bed, the object can start to curl and/or lift itself away from the printing surface. Usually, a higher difference between the printed object’s temperature and the ambient temperature leads to more warping.
How to fix/prevent warping:
Make sure the printer operates in a warm room with a stable temperature, or use a printer enclosure. Sudden gusts of cold wind (e.g. from an open window) or cold draught are common causes of object warping.
Keep in mind that certain materials are more (or less) difficult to print – e.g. ABS has a much higher tendency to warp and the best way how to print ABS is to use an enclosure. If you need to print ABS without an enclosure, make sure that the printer’s fans are off and the room where you print is well heated.
Also, another cause of possible warping is a poor first layer. If you are experiencing a lot of warping issues, re-run the First layer calibration and check your Live adjust Z values. Also, consider using a Brim (in Slic3r PE), which is a function that will add a brim around the printed object, increasing the contact surface and improving the adhesion.