When MultiBit Classic encounters a problem you will often see an error message. This can be in a small dialog box or written in the status line at the bottom of MultiBit Classic. Messages written to the status line are also repeated in the 'Messages' tab.
Like clues in a detective story you can often work out what the original problem was from these error messages. See if your error message / exception is listed below - if it is click on the link for more information.