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