Hi Susantha
The problems may relate to a file corruption and the old versions are no longer supported by MYOB. If you can log in, then click on File, Verify Company File as this may show if there is a problem. If so, then sometimes you will be able to work out how to fix it, or if not, you may have to restore a previous un-corrupted backup and re-enter your data.
There is a company called FGH Tables that can sometimes (not always) fix these problems for a fee.
The error message about being locked usually relates to a Lock0001.flk file that should automatically be deleted when you log out of MYOB but sometimes isn't. Try turning your computer off and on and then search for this file before you do anything else and delete it if it's still on your system.
Cheers
Liz