PG 1553+113 is a well-known TeV blazar, whose redshift is still uncertain (z>0.4, ). The source has been monitored in VHE gamma-rays by MAGIC since February 2005 and has been detected on a regular basis in a quiescent state with modest flux variations, lying in the range from 4% to 11% of the Crab Nebula flux above 150 GeV (Aleksic et al. 2012). The source has remained as well in quiescent state in the Fermi energy band since the beginning of its operation. However, in March and April 2012, strong VHE gamma-ray flaring activity was detected with the MAGIC telescopes. The flux at ∼100 GeV reached an unprecedented level for PG 1553+113 (∼Crab Nebula flux). An intense multiwavelength campaign was carried out during the whole period in optical by the KVA telescope, in optical-UV by Swift/UVOT, in X-rays by Swift/XRT, in infrared by the REM telescope as well as in gamma-rays by Fermi/LAT. The source was observed for ∼18 hours with the MAGIC telescopes and due to the high state of the source, the high statistics of the detection allows a very detailed spectral study. The observed spectrum cannot be described by a simple power-law. This is the most distant blazar for which spectral curvature has been found. This spectral feature can have an intrinsic origin or can be produced by the interaction of VHE gamma-ray photons with the Extragalactic Background Light (EBL) photon field. Considering a redshift of z=0.4 the spectral curvature is most likely due to the absorption effect by EBL. Therefore, the VHE observations together with Fermi simultaneous observations provide a unique set of data to probe the intrinsic properties of the source, the EBL and the Intergalactic Magnetic Field (IGMF). In addition, the multiwavelength behaviour is interesting in order to test the standard emission models. In this talk we will present detailed results of the MAGIC observations together with the results of the MWL campaign, as well as the implications on EBL and emission models.