If you are using Windows (Vista or later), ExpressAccounts data are stored in ProgramData folder (it may be hidden). For example, on my PC it's stored at:
C:\ProgramData\NCH Software\ExpressAccounts
1) Exit 'Express Accounts' program
2) Delete above 'ExpressAccounts' folder and including all subfolders. OR (just to be safe) rename it to something else like 'ExpressAccountsBackup'
3) Run 'Express Accounts' program. It should start up with wizard to enter 'Company name' and other start-up details.
Hope this helps!