A roller coaster goes up and down, the stock market is different.
Some stocks never return to previous levels.  Some companies sell off
part of their assets to continue.  From then on they are clearly not
worth as much.  Such is the case with so many financials - they made
bad investments and the financial data that the high stock prices were
based on were incorrect.  Consider ENRON.  More like a slide than a
roller coaster.

A stock is worth today what it is worth today.  It has no obligation
to go back to where you bought it.  Review your holdings regularly.
Ask yourself, ignoring the purchase price, whether you would buy it at
the current price, considering all the other investments you could
make.  If not then sell whenever the price seems relatively high and
buy something else when the price seems relatively low.  In this
market buy and hold is not the best strategy.