This paper shows a detailed description about an uncommon electric submersible pump (ESP) failure in Candelilla-5 well. This catastrophic failure generated fishing operations to recover the well, and additional cost for the well service. Candelilla-5 is located at Guataquia block, in the Llanos Orientales basin in Colombia. It is an oil producer well, 600 BOPD, 19.7 °API, an ESP as artificial lift system, at depth of 9482 ft TVD, with runlife of 173 days. The ESP suffered a suddenly stuck pump shaft during the operation; several start attempts were made, with no success. It was determined by the given evidence in this and the neighboring wells, that the probable cause was the presence of calcium carbonate (CaCO3). The electrical measurement of the all ESP system was in good conditions. Aiming to remove the CaCO3, two barrels of hydrochloric acid (HCl) at 15% concentration was injected into the pump, but it did not work, so a rig service was programmed. During the pulling of ESP string, only the tubing string plus pump discharge head was recovered. All the screws of pump discharge head were broke and the rest of ESP remained into the well as a fish. A Root Cause Failure Analysis (RCFA) was developed. The following methodology was used: (1) fractography analysis that showed a fracture plane between head and screw's body, (2) elemental chemical analysis using optical emission technique with spark source to find the screw's composition, (3) macro-attack testing for detecting residual stresses, (4) microstructural analysis to characterize the screw's material and fractures (5) microhardness profile to identify any screw's hardening, (6) X- ray diffraction in direction to identify incrustation and corrosion products and (7) hydrogen evolution test using HCl solution at 7,5 % and 15%, with the purpose of determining the hydrogen effects on steel properties. Once the RCFA was successfully completed, it was determined that the screws were fractured mainly because of three reasons: (1) High tension zones created due to high stresses during pump service, (2) the thermally treated material and (3) the presence of an acid solution in the environment, HCl, inducing an accumulation of hydrogen on the high tension zones, giving as a result the screw's hydrogen embrittlement, phenomenon responsible for the failure of the ESP from Candenilla-5 well. This paper aims to give details of the methodology, analysis from the root cause of the failure, feedbacks and recommendations to improve operations development on ESP.