You would have been prompted to do a backup before you moved to the new year.  So just restore this backup and continue on with what you were trying to do.  If you did not do a backup, there is usually one done for you anyway, so just look under My Documents\Simply Account\Backups\CANYear for the appropriate file.  Copy the *.sai file and *.saj folder with the same name and remove the date and time from the names.

There is no way to reverse a year end transition within the program, data repair is the only way if it has to be done.

However, depending on when your year end really is, the program does allow you to post to the current and previous years, so if your year end is between now and Dec, then it might not be that bad, as you can back date entries.

No.  

You can restore from backup if you need to re-open the year before last.  

i.e. if the new fiscal year ends in 2015, 2014 just became 'last year' and 2013 is permanently closed.

If you need to enter more transactions in 2014, just change the '[  ] do not allow transactions before ____ ' in Setup,  or uncheck it.  

The normal workflow in the newer versions of Simply Accounting / Sage 50, is to open the new fiscal year on the day it starts.  In the DOS versions from last century, you might have wanted to keep two files running until you 'really closed' the last year, or used some other contortionate work-around.