It may have just accidentally been shrunk to zero size. Position
your mouse at the bottom of the screen until it turns into a two
headed arrow. Then click and drag it upward.
I can still access the start menu by hitting the Windows
key and I can change between programmes by using alt-tab.
I have looked at the settings for the task bar in the
contol panel and these are fine.
Windows has all the latest updates and the system is free
from Virius's (or should that be virii?)