jozxyqk's answer is correct, but it is fairly drastic. It will blow away all of your toolbar customizations, app favorites , etc.
Depending on the cause of this issue in your case, you may be able to fix it by editing your plasma-org.kde.plasma.desktop-appletsrc file and then performing
kquitapp plasmashell && kstart plasmashell
One cause I've run into is the buggy multimonitor functionality sometimes "losing" the taskbar from my laptop screen after I've connected to an external monitor and later disconnected (this was in plasma 5.5 or 5.6, I think). In this scenario, I've found the problem to be the lastScreen setting in the taskbar configuration. To fix this, find the section of text in plasma-org.kde.plasma.desktop-appletsrc that looks like:
(Some of these lines may vary in your configuration. In mine, this section was only a dozen lines down in the file, but I have no idea how much that may vary.)
Setting lastscreen=0 fixed the problem for me. I assume that 0 represents my laptop screen, and 1 is the external monitor.
If in doubt, you can always perform the procedure in jozxyqk's answer, and compare the newly generated plasma-org.kde.plasma.desktop-appletsrc with your saved plasma-org.kde.plasma.desktop-appletsrc.bak before restoring an edited version of your saved copy.