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BL Lacertae (BL Lac) is categorized as TeV blazar and considered as apossible source of astrophysical neutrinos. In 2020, the brightest X-ray flareever detected from it. A detailed study can answer many puzzling questionsrelated to multiband emissions and fast-flux variability often seen in thiskind of source. We have performed the temporal and spectral analysis of thebrightest flare. The variability is characterized by the fractional variabilityamplitude and the variability time. We found that the source has crossed allits previous limits of flux and reached the maximum ever seen from it inoptical and X-rays. It is highly variable in X-rays with fractional variabilityabove 100$\%$ (1.8397$\pm$0.0181) and the fastest variability time of 11.28hours within a day. The broadband light curves correlation with X-ray suggest atime lag of one day. A broadband SED modeling is pursued to understand thepossible physical mechanisms responsible for broadband emission. Modelingrequires two emission regions located at two different sites to explain the lowand high flux states. A significant spectral change is observed in theoptical-UV and X-ray spectrum during the high state, which eventually leads toshifts in the location of the synchrotron peak towards higher energy,suggesting an emergence of a new HBL component.

Broadband study of BL Lac during flare of 2020: Spectral evolution and emergence of HBL component