BL Lacertae (BL Lac) was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. Simultaneous or quasi-simultaneous observations were taken at radio (University of Michigan Radio Astronomy Observatory and Metsähovi Radio Telescope) and optical (Whole Earth Blazar Telescope [WEBT] collaboration) frequencies, in X-rays (BeppoSAX and RXTE), and at very high energy gamma rays (HEGRA). The WEBT optical campaign achieved an unprecedented time coverage, virtually continuous over several 10-20 hr segments. It revealed intraday variability on timescales of ∼1.5 hr and evidence for spectral hardening associated with increasing optical flux. During the campaign, BL Lac underwent a major transition from a rather quiescent state prior to 2000 September, to a flaring state for the rest of the year. This was also evident in the X-ray activity of the source. BeppoSAX observations on July 26-27 revealed a rather low X-ray flux and a hard spectrum, while a BeppoSAX pointing on 2000 October 31-November 2 indicated significant variability on timescales of ∼a few hours and provided evidence for the synchrotron spectrum extending out to ∼10 keV during that time. During the July 26-27 observation, there is a tantalizing, although not statistically significant, indication of a time delay of ∼4-5 hr between the BeppoSAX and the R-band light curves. Also, a low-significance detection of a time delay of 15 days between the 14.5 and 22 GHz radio light curves is reported. Several independent methods to estimate the comoving magnetic field in the source are presented, suggesting a value of ∼2e_B ^2/7 G, where e_B is the magnetic field equipartition factor with respect to the electron energy density in the jet.